3 fingers, 3 toes... The devil.
I've always simply viewed it that women are there, other ethnic groups, and the Empire had some near-humans involved (IE: Thrawn, perhaps some other groups) we just don't see them in what we ultimately see in the films. I've always just taken what the films show with a grain of salt that way.
I agree too, actually, that I don't think Palps is a xenophobe so much as he just doesn't care and lets those under him decide to use an anti-alien agenda to A) wrangle the galaxy somewhat, B) get payback on alien races that caused trouble for him in the recent past or in the Clone Wars, and C) because it helps unite the greater population of humans towards the new Galactic Empire via propaganda... Basically they say, "Look what these aliens caused... Look what they did!", and blah blah blah. Many humans across the galaxy agree, saying look at the Trade Fed, Banking Clans, etc. And then through some twisting other species that tried to help the Jedi are also vilified, like Wookiees, Mon Cal, Quarren... Some aliens would try to assimilate maybe, while others would openly fight back. In that time from ROTS to ANH, I feel like there's a growing disparity in how humans view other species, and also how other species may view humans as well.
And TOTALLY agree on Leia... 3P0 is being 3PO I think, but Leia, that's really horrid I felt. Funny, at the time, but when you cut away and over-analyze Star Wars it's so f'd up really.
I really do feel though that the Empire has a lot more variety to it than we see, even home guards for planets of aliens trying to integrate into the new Empire... I think as an organization they'd know they couldn't be completely xenophobic towards a galaxy that vast. They'd do things subtly, at the very least. I think they'd do the same with other races of humans and women as well, if they were doing it. Keep women out of the Stormtroopers, or a glass ceiling for most... Thrawn and other similarly held back people would only get to the level of power they attain because they impressed the right people... which is actually Thrawn's story. He reaches Grand Admiral status directly from Palpatine. If he were relying on someone else with the power to grant that rank to him, he may well never have gotten it.
Yavin IV, total sausage fest indeed... Leia's like Smurfette.
Again, grain of salt and all, I figure women were there... There's enough jumble in those ranks, I feel sure women were in there, even if women weren't actually in there. I figure a few aliens were sneaking around in there too. Just nothing larger and/or significantly smaller than the average human male I guess.