Ive a pretty strange empire.
The first race that I had contact with early in the game declared war on me and actually was stronger but I won because I destroyed their shipyards. They couldnt produce more ships while I was able to bring in new fleets. That was my first vassal which I eventually included into my empire (now they are the backbone of my ground troops).
Since that I have defeated many other species and included them all into my empire. At some point I allowed them to have the same rights as my species.
They can vote and for a few centuries the president hasnt been a human anymore.
I even have robot planets and all species of the empire seem to have no problem with that.
At some point I even uplifted cockroaches from a radioactive world so now ive intelligent cockroaches which can colonize other radioactive worlds.
At some time a species from another universe invaded the territory of one of my biggest opponents. Many system fell under control of that invader and I used that to fight this invader back and take control of all the planets. There was a nice vacuum of power and after my victory all species of the galaxy were friendly to me for a period of time. I made many contracts which allowed me to use the natural resources of other species.
Eventhough I lost MANY ships it was worth it.