ARM - Armor
Considerung amounts from 10 (private and non-combat-ships) to 5-10% for other ships and 15-20% for bases (20% for the defensive base, maybe even star base).
I personally think that that's too much armor. Railguns aren't that big a threat due to their pitiful range and lowish damage, graviton beams ignore it, and shields protect against everything else, and armor components cost a lot of steel. A couple of armor plates are plenty for civilian and noncombat ships. My small warships get maybe 5 armor plates, mid-size and up gets ~5 + whatever fits in the space I'm not using for anything more useful.
CMP - Commerce Center
Most likely inevitable on Space Ports, Mining Stations (guess they're selling to neutral merchants etc.) and Star Bases. Essential for Resort Bases. What else could need these? And does the bonus stack, further increasing income and if so, is it capped?
The first of these that you add is the only one that gives an income bonus. Anything that you expect to see freighters or other civilian vessels dock at is something that can use it.
CMP - Targetting System
Must-have on anything with weapons, might be less useful on pure carriers without weapons, though Fleet Targetting System should still be more than useful on those (especially if it's a flagship). ONLY ONE COMPONENT USEFUL. I think TS & FTS stack with each other?
CMP - Countermeasures System
Probably good for anything to prevent getting blown up. Used to build these in anything (ships, bases). Fleet Countermeasures also good for flagships / carriers. ONLY ONE COMPONENT USEFUL. I think CMS & FCMS stack with each other?
As far as I know, the fleet version stacks with the ship version, but neither stacks with itself. It's not really a bad idea to carry a pair of targeting systems and a pair of countermeasures systems to give yourself a little insurance against damage, but it's not going to increase the resultant damage reduction or accuracy enhancement while both are in operating condition.
CST - Repair Bots
I've heard any bot beyond the first won't supply any bonuses, though using at least twice provides redundancy, repairing each other in case one being destroyed. Is this correct?
Stacking them doesn't increase the damage reduction. I don't know if multiple Repair Bot modules allow you to repair multiple components at once or not. They will repair other destroyed repair bot components.
CST - Damage Control Unit
Must-have for warships, maybe to expensive to use them in everything, can't remember the cost right now. Guess more than one's useless (except for redundancy).
These should be replaced when you have Repair Bots, as Repair Bots provide a better damage reduction and repair damaged components but do not stack with Damage Control Units.
ECL - Energy To Fuel Converter
Pretty heavy, so might only be useful on bases. Though I remember having read that a single Gas Mining Station easily provides more fuel in the same time.
Not a very useful component. Mining stations should generally provide more fuel in the same amount of time unless you're mining a very poor source or maybe have only one unupgraded extractor. Could be useful if you wanted to put a base in an empty system or a resourceless gas cloud to act as a forward base of some type, but won't really be able to support the same kind of fleet traffic that a resupply ship or gas mine over a proper fuel source could. It also gives both types of fuel, so it wastes a bit of your cargo space if you only need one fuel for your fleets, and standard practice is to use a single type of fuel for all military ships except when you find a sufficiently useful wreck that uses the other fuel.
You could also theoretically use these on Resupply Ships to produce fuel instead of having to find worlds that have the right kind of fuel, but it carries the disadvantage of producing both kinds of fuel, and the unused kind is just going to build up in the Resupply Ship's cargo bays if nothing is taking it.
CST - Construction Yard
Does more than one CY make sense on a construction ship? I'm not sure if both can construct one base at the same time possibly adding up their construction speed. Because they are somewhat heavy I'd consider building Construction Ships with more than one not before mid-game.
Only one construction yard can work on a given project at any one time. Sorry.
ECL - Energy Collector
Must-have on anything, right? Prevents fuel usage while not travelling. Could also power complete bases, only mounting one reactor to be able to save the design. Though more than inefficient and expensive on bases with excessive energy usage. Do you suffer any disadvantages of you use ECs for a base, having a negative static energy coverage?
Static energy use which isn't covered by collectors gets picked up by reactors, which eat into your fuel stocks. Not a big deal for something like a spaceport, which gets lots of freighter traffic, or a gas mine over a fuel source, but a station that lacks fuel and doesn't have energy collection at least equal to its static power draw isn't going to be able to fire its weapons or power its shields. I would also not personally recommend allowing your spaceports and gas mines to eat up locally available fuel to make up for a lacking energy collector array, as it's just one more thing that can contribute to an area's fuel shortages, which can be enough of a headache already.
EXT - Mining Engine & Gas Extractor
According to a thread (can't post links yet, thread id: 3634839) the Gas Extractor gathers three times the resources of a Mining Engine. But due to everything usually needing fuel this is kind of necessary.
How much of these do you use on your Mining Ships and Stations (I'm using 1 & 3 respectively)?
EXT - Luxury Resource Extractor
I'm using equal amounts of these and MEs & GEs. I was once thinking about building a separate base focused on Luxury Resources and sparing these parts on the (Gas) Mining Stations and vice versa. But I fear this could increase maintenance costs beyond any additional income.
The computer designs use 1 and 3 on mining ships and stations, and I think slightly fewer luxury extractors on stations. I typically let the computer do what it wants with the civilian ships, aside from ensuring they get the most out of their hyperdrives, but if I decide to design them myself I usually up the mining ships to two extractors of the appropriate type and leave it on one luxury extractor. Mining station extractors I typically leave alone. If your extractor technology is low enough, you'll see a minor benefit to adding a fourth extractor, but I believe that someone looked into this a while back and found it wasn't economically useful.
HAB - Colonization Module
Pretty heavy, just one question: Anyone using more than once on a single ship of these?
Doubt it.
HAB - Medical Center
Must-have in Space Ports. As I use to build a SP on every colony any other bases on a colony equipped with this wouldn't provide an additional bonus, right? Do these make sense on military ships without Troop Compartments? According to the help they allow onboard troops regenerating faster, so they should also be useful on Troop Transporters. Do multiple MCs make sense?
HAB - Recreation Center
Also a must-have in Space Ports. Same question as above. And do they provide anything useful installed on ships or other bases? ONLY ONE COMPONENT USEFUL.
Multiples only make sense from a redundancy perspective. I'd consider adding these to defensive bases, since I generally wouldn't build a spaceport over a colony if there's already a spaceport in the same system, though how many spaceports you build is up to you.
HDR - HyperDeny
Those need to be handled with care. They prevent any ship jumping, even your own. One of the later technologies (don't know from memory which one) doesn't have this disadvantage any more. I was considering using them on my more important bases/stations. Guess you only need one of these.
They're good on stations if you're looking to kill things that attack the station rather than just driving them off. It can also be useful to force decisive fleet actions, though a ship that thinks that it's losing is going to shut off any HyperDeny components it has so that it can run away better, so that's one case where multiples (though not on the same ship) could be useful. The Interdictor version pulls stuff out of hyperspace in addition to preventing it from jumping, so it can be useful for breaking up fleets trying to attack a base or, in combination with long-range scanners, intercepting stuff in deep space.
MNF - Plants
I've seen them in increasing numbers on the Space Ports. How much of these do you need to support a steady construction speed?
There have been arguments about this, but the most convincing information I've seen suggests that one of each plant type is sufficient for at least 30 construction yards going nearly full bore. Don't know where the thread is, now, but maybe someone will post a link.
SEN - Scanner Jammer & Trace Scanner
How useful are these? Just one of these or more on a single ship?
I wouldn't bother with these unless you're trying to reduce the number of pirate freighters (trace scanner) or trying to avoid losing your own pirate freighters (scanner jammer). There's no real benefit in them for military ships that aren't being used for customs duties or for legal civilian freighters, and there's not any real benefit to the 'scan the component status of enemy ships' aspect so there's also no benefit to countering that.
SHD - Shields
I use 1/10 of shields compared to armor (rounded down if need), making them equal in weight. So a ship with 30 armor components would have 3 shields. They're pretty much some extra protection if they're not up against weapons bypassing them. Due to this fact I've been thinking about reducing shields by maybe 50%.
Unless you're facing a foe who went very heavy on the railguns, I would strongly suggest using a shield generator in place of every 10 armor components you have beyond the tenth or so. The majority of weapons in the game have to penetrate shields before they can do anything against the hull and armor, railguns and gravity weapons are short-ranged, railguns are ineffective against even small amounts of decent armor, and gravity weapons have a chance to hit any component on the ship regardless of the presence of armor or shields. Bottom line is that 100+ shields and 0.5+ shield regeneration for 10 size is more useful than 10 armor plates for the same size, unless you just don't have much/any armor in the first place.
STR - Cargo Bays
I can't see any reason to install them on anything else than Space Ports, Freighters, Construction Ships and (Gas) Mining Stations. If you got fuel in a cargo bay and your fuel cells are empty, is the fuel transferred to the fuel cells - assumed the fuel type is the same?
As far as the self-refueling goes, resupply ships will but I don't think anything else does. However, non-freighters and non-construction ships will not typically pick up resources and store them in cargo bays whether or not you have cargo bays in the design. Adding these to a military ship is just wasted space.
STR - Troop & Passenger Compartments
How much of these are good to have on the according ship types? How much Passenger Compartments are good to have on Resort Bases?
Can't answer for the passenger compartments. Troop compartments I like to add in numbers that get me a (more or less) integer multiple of 100 capacity, because units cannot be split across multiple transports, infantry units take 100 transport capacity and armor units take 200 transport capacity. I think planetary defense units take 400 or so capacity, though I'm not sure, and I can't recall what special forces require. Regardless, if you want to move armor, you need at least 200 transport capacity per transport (two of the basic troop compartments, or one of the fully-upgraded massive troop compartments), so that's a minimum of two compartments. I personally like to have 5 or so compartments on my transports, and keep the total size of the transport around 300. Other people like to have massive troop ships that can go in while the space station is still up and drop a massive army all at once (which has its advantages, I'll admit).
WGR - Tractor Beams
Just a simple question: Pulling or pushing enemies most likely depends on the combat settings, if the ship is trying to standoff it'll push them away and otherwise pulling them, is this correct?
The decision to push enemies away versus pulling them in appears to depend on whether the ship with the tractor beam thinks that it is stronger than the opposing ship and winning the fight. If it thinks that it's weaker than the opponent or has triggered whatever flee condition is set, it will push the opponent away. Very useful component for killing pirates or drawing unlucky attackers into point blank range for your computer-designed large spaceport's 30 or so blasters, and also makes it a little harder for enemies to finish off a damaged ship that has some of these equipped.