I think there were a few species of aliens that worked with the empire, at least in the beginning. Once it became apparent that the empire was going to eventually eradicate or enslave them all, most jumped ship. Even the Chiss. By the time of the second death star, you would be hard pressed to find entire alien species amongst them. Individuals sure, but an entire species would difficult to explain or come by. Less the Noghri or other special, ignorant, cases.

Politically, a lot of false hope and broken promises, would have driven them away. Like some believed that by working with the Imps, they would spare their home world/species from the cesspool, but as time goes by they realize that living under said conditions isn't really an option.

That's how I'm running it, at least.

I really don't care about EU stuff. From the Prequels you can see that most of the Senate is composed of non humans. The Senate was responsible to give powers to Chancellor become Emperor. It took 20 years to close the senate. Before that, I assume the Emperor needed a good lot of support from the Senate.

I think you guys are seeing it the wrong way. The Empire took care of military and security for all the planets in the Republic. Non humans liked it. The Empire used clones (and then non clones) for their forces.

The humans are cannon fodder and the other races prolly liked. They compose the "Mighty" Imperial Military (Army, Navy, Troopers) to secure the empire from external threats (outer rim planets and orgs)

Local security forces ("State Police", if you will) will be composed of their kind, of course.

We must forget most WEG nonsense from years before the Preqs.

That's my 2c.

I dont care about prequels made by G.Lucas who may tomorrow decide that it was Jar Jar who shot Greedo and say that he had such vision from the beginning just could not implement it .

He still would have done more then Boba. Just sayin...