Advantages/ Disadvantages having other alien races on planet?

[Novaliz]

What are the advantages or disadvantages allowing for example other alien races live on my planets?

[Siddham]

Each race has certain racial bonuses - which you can check in the Galactopedia
Having them in your empire can give you some level of those bonuses
You can see any bonuses you have listed in the Empire Summary screen

[SirHoraceHarkness]

Yep. The more races you have under your control the better bonuses you will enjoy. The trick is keeping them happy so they don't endlessly revolt.

[Retreat1970]

Per the Galactopedia:

"Once your empire contains 10 million citizens of a race, you will begin to receive a small portion of their racial bonuses. If a race ever grows to the point where it comprises 20% or more of your empire’s population, then you will receive its full racial bonuses."