Chewie aside, the rebellion didn’t have an alien in their ranks until ROTJ.
Now I’m not going to argue that the Emperor isn’t different looking because he’s an evil wizard - that’s obviously the intention. But there really is nothing in the film to suggest he couldn’t be an alien. In fact, he looks quite a bit like some of the rebel aliens (prune faces).
I like the Empire-as-specieist idea, and it fits, but it’s not a part of the films’ text. He’ll who’s to say the storm troopers aren’t aliens?
Are there any black Rebels before ROTJ? (I don’t count Lando)