I'm gonna use the description to explain each choice, so...
Robert Downey Jr for Julio - It's already been said here on FJ before so I don't even need to explain this one do I? He fits perfectly
Bradley Cooper for Miguel - I imagine people might disagree with this choice because most people would want Johnny Depp instead, but, If you've seen Bradley Cooper in any other film apart from The Hangover's, he's really ******* good. And plus as we know from The Hangover films he can bounce off other actors really well, and I think the pairing of him and RDJ would be hilarious.
Rosario Dawson for Chel - I'm going to be honest, I don't have a clue who she is but my knowledge of Latino actors is ******* terrible and she looked like a good fit, I would have said Michelle Rodriquez but I used her in Atlantis. Please recommend someone else if you know a better actor for Chel!
Danny Trejo for Tzekel-Kan - He looks scary as **** and is Latino. He might be a bit too old but I don't really think that's a problem.
Danny McBride for Hernan Cortes - This is definitely a choice some people might think is retarded but I think Danny McBride would work really well, he's brilliant at playing evil cunts (This is the End, 30 Minutes or Less, Pineapple Express) and I think it would be hilarious to have scenes of him following Julio and Miguel and just getting really pissed off.
Omid Djalili for Chief Tannabok - I'm not sure if you guys from Murica know who he is but he's pretty ******* funny, and he's the only fat tanned dude I could think of. And with some make-up and a fake tan he would work pretty well as this role I reckon.