Considering the size of the Empire and the number of species in it, I pretty sure it couldn't afford to be racist. They might have a human superiority complex, or maybe they just have an over abundance of humans in the military are a result of the Clone Wars and most of the materials from that army were made for humans.....or actually just one human, who had millions of clones made of him. But the Empire is full of aliens, and some in high positions, just not many in the military (probably because of the Clone Wars and the old Grand Army of the Republic again being made up almost exclusively of humans). Now that's not to say the Empire's representatives can't be racist. Some definitely are, but most of those are probably due to stigma and other drama caused by the Clone Wars, and some species that fought against the Republic, and some that harbored the Jedi post-war. Those races got the hammer dropped on them by the Empire (Wookies, Mon Calamari, and a few others). Others, like the Goenosians, were exterminated to keep secrets (Death Star). And others, like the Twi'leks, just want to be free, and continue fighting against the Empire, which is totalitarian anyway.