Furious, keep in mind that the large imperial and columbian mammoths were large savannah animals. A good rule of thumb is that the bigger they are, the more open their habitat is likely to be.
DPII will do as he sees fit, but I'm just suggesting the mastodon as a flavor alternative against other elephants, and given that most of central america is forested, a smaller mastodon seemed more fitting. Likewise, if you're gonna get loose with history (why not, for a non-historical generic game) then an imperial mammoth might be more suitable for a north american plains indian (like that guy on the "Iroquois", er, Souian mounted warrior stock unit).
Whatever the correlation, smaller forest elephant species seem to be more domesticable than their larger plains relatives, including the cyclotis loxadont being marginally easier to get along with than the more common savannah loxadont species.
For ease of animation, why not just put an Aztec, etc., form of the howdah on the backs of various beasts? And it then would conform with the stock War Elephant without contradicting the sizing of the other mounted (horse) units.
Just a(nother) suggestion.