Anti-alien Empire Origin

[The Romulan Republic]

I'd say its pretty easy to conclude that the Empire is bigoted due to the lack of pretty much any Imperial character who isn't a while male human in the Original Trilogy.

Palpatine did work with aliens in the prequels, as someone already mentioned, but that simply means that Palpatine was pragmatic enough to use anyone who could be politically useful.

[Anacronian]

I can not imagine how difficult it would be to have a multispecies version of something as big as the Imperial Navy - imagine just having a Navy with 10 species all with individual needs and preferences hell just imagine the headache of producing uniforms alone - now imagine that on something as large as the Imperial Navy with the thusands of species in the galaxy.

It could be that whatever bigoted feelings there is in the Imperial Navy grew out of the fact that only humans serves in the Navy (out of necessity) rather than the bigotry was a starting point.

[The Romulan Republic]

Yet the Rebel Alliance and the separatists have a multi-species military. And while the Grand Army of the Republic was pretty much one species (aside from the extremely diverse Jedi), it is an army of clones and a precursor to the Imperial military.

But I suppose it would be hard to have numerous very different species because they wouldn't be able to effectively use the same equipment. I suppose the separatists got around this by having standardized droids as the rank and file. There's also the fact that the CIS was an alliance of different factions rather than one force. And the Rebels mostly had just one species in each area, at least in the films (i.e. humans as fighter pilots and ground troops, Mon Calimari as capital ship crews, and Bothans as spies).

[Eleas]

This is EU, yes, but Aaron Allston's X-Wing novels made a pretty clear distinction about pro-Human sentiments being a Core-world thing rather than a specifically Imperial one. If you think about it, it makes perfect sense for the Empire and Palpatine to capitalize on pre-existing supremacist notions among the wealthy in order to create a shared identity.

[Purple]

It could also be a result of the clone wars. Yes, there were aliens fighting on both sides but if the core worlds are predominantly human and the CIS is predominantly alien (the later of which is cannon) it would be relatively easy for a good statesman to spin things in such a way that promotes hatred toward aliens in a bid to unite the population by creating an imaginary internal enemy like the nazis did.

[Havok]

Yet the Rebel Alliance and the separatists have a multi-species military. And while the Grand Army of the Republic was pretty much one species (aside from the extremely diverse Jedi), it is an army of clones and a precursor to the Imperial military.

But I suppose it would be hard to have numerous very different species because they wouldn't be able to effectively use the same equipment. I suppose the separatists got around this by having standardized droids as the rank and file. There's also the fact that the CIS was an alliance of different factions rather than one force. And the Rebels mostly had just one species in each area, at least in the films (i.e. humans as fighter pilots and ground troops, Mon Calimari as capital ship crews, and Bothans as spies).

Yes, but the ships were created and crewed all by Mon Calimari which could be racist the other way.
Other than Lando's co-pilot Nien Nub(?), which is just a play on Han and Chewie, there really aren't any other aliens shown.

ANH does more for promoting racism in the galaxy by showing the Jawas trying to rip off the hard working human farmer, the Sandpeople attacking our blonde haired blue eyed human hero, having the worst wretched hive of scum and villainy being packed with aliens and again, attacking our blonde haired blue eyed human hero, than showing the Empire, who blows away a planet of white people, as all stocked up with humans.

[The Romulan Republic]

The capital ships being all crewed by Mon Calimari is kind of my point. The Rebels had a diverse military, but they mostly had one species in a given area, perhaps because of the difficulty of having a multi-species military being discussed.