I looked at the first three movies again recently, and if there were any non-humans in the Empire, I didn’t see any. I suppose it’s possible that some Stormtroopers are non-human, but not one person wearing an Imperial uniform was visibly alien. All aliens were either Rebels or independent of both the Rebels and the Empire (Ewoks, bounty hunters, Jabba and his gang, the cloud city, etc.).
Could this have been deliberate? Did Lucas consciously decide not to include aliens in the Empire as a subtle clue as to the Emperor’s motives (the Emperor wanted an all-human galaxy and his ultimate goal – other than power – was genocide of all non-human species) or was it just a lack of time and money that kept him from putting aliens in Imperial uniforms?
The movie are never very explicit about this, but most of the books and comics have painted the Empire as pretty human supremicist. Not exactly Nazi-final-solution supremicism, more of a Jim-Crow, seperate-but-unequal kind of supremicism.
From the books I’ve read, the Emporer himself was a huge bigot and wouldn’t let any non-human non-males in any high positions. The only exceptions I can think to this is the bad-ass Admiral Thrawn and some female admiral from another book who was appointed by Tarkin.