The thing with elves is, I think, that logically they actually fit rather well into the position that indigenous peoples had during the early colonial period. They have extremely long lives, and because of this, they are easily pushed back by short lived and more prolifigate races. Also, because of those same long lives, they lend themselves well to representing ways of life that have basically stayed the same for thousands of years. At the same time, later in the colonial era it became rather common for there to be substantial groups that had mixed background. I do not know how exactly this played out in Mexico, but up here in Canada the Metis are well known and a major cultural group. Thus how I saw half-elves should absolutely have a prominent place in such a setting. Obvious elven and half-elven cultures in a Wild West setting should not be mere copies of real world ones, but I think that matching roles played in the setting lore is appropriate.
I could live without tieflings and dragonborn being common races in the new edition, but I expect that will be a vain hope. Thus, my suggestion that they would be better to have some relationship with the elves as mystically endowed "priest-class" races. That gives them a role in the setting that fits thematically, while keeping them rare. Keep in mind too, that the tropical regions adjoining the fantastic "Wild West" need not be a focus of setting details or gameplay, but I think there should at least be the feeling that they are out there.
Dwarves are obviously going to be the big time mining experts in such a setting, which is why I think they would work better as one of the invaders as opposed to one of the natives. If I were to establish a firm place for halflings as invaders as well, I think I would assign them the role of the pilgrim and missionary types who fled religious and racial persecution to establish themselves in a new land. And finally, gnomes as Inca was mostly based on what I was thinking of "Which of these races would be cool as people who live in high mountain cities like Macchu Picchu?" It would give them a bit of a distant quality as well. Alternatively, instead of giving the devas/aasimar some role in the elf/tiefling society, perhaps they are the ones who live high in the mountains, with gnomes making up sort of lost forest tribes.