ORIGINAL: z1812
2. Can one choose their own units for this type of Battle? Could one choose the force mix for both sides?
The RBG picks the units for you based on the best data for the equipment distribution dates that we have. The RBG produces a distribution RBG_Z.txt file for you to look at after the generation process if you like. This lists the % probability of a given unit to appear (it is essentially a large table)
However this is all shaped by what preset you have chosen. You can randomize the presets if you like.
A preset defines data about a particular "outfit" during the war (eg a company, regiment, division etc). Specifically the name of the outfit, the selection of units available to that outfit, naming conventions used at the platoon level, and it also sets the force mix % for you. If a preset defines say a panzer div which used PzIIs early in the war but Panthers later, the RBG still applies a date distribution function to the list of potential units so that you don't get any ahistorical date mixups.
You can add your own presets to the library of available ones pretty easily. Hopefully people will share their own presets with others as well to help enrich the community data driving files for the RBG. The more and better the input driving files for the RBG the better the generated battles will become. The other driving files being the map template, setup zones, battle plans themselves.
The preset idea was developed to allow random battles with some historical flavor like "1st panzer div vs 7th tank corp" rather than just "grey vs red"
You can also just choose a default preset for each side. This is just a generic all units, default naming conventions, default mix % "grey vs red" preset.
It is also possible to change the force mix % bars after you load a preset if you want to further tweak them to your liking.
Once you are in the game and during purchase, you will also have some ability to adjust your platoon strengths (you do have fixed points, so if you strengthen one platoon it is at the cost of weakening another).
I hope that answers your question.
3. Can the battle be saved as a scenario?
The battle is saved as a scenario called "Random". You can play it time and time again if you like. If you generate a new battle then the old one will be overridden. If you want to keep a particular generated battle, then simply copy the battle to something else.