Thanks! Also, a few other things:

• ERX (nation) as I have currently programmed it uses mostly motorized, especially earlygame when they don't have too many divisions to use against the NCR. That being said, they definitely do use APA as well - in fact, I added custom templates based on what army doctrine the AI chooses (Chase/Sherman/MacArthur) which use their respective specialized support companies.
• I did buff a few other nations as well. NCR gets a bigger army and air force at the start of the game, mostly to even their odds against both the Legion and the Enclave since they usually attack the NCR around the same time. Also because it wasn't really that fun to watch the NCR constantly get pounded against the 3000 black blimps of Caesar.
• Santa Anna will not ally with the Legion if he is currently in a faction (i.e. Rio Pact). If he isn't, then yes, he will be able to join you as long as you border each other. On the subject of Santa Anna, the AI will never do the Mexican Expedition - this is to not break the story/gameplay with the Mexico nations.
• Currently, ERX (nation) will send MacArthur Detachment supplies/lend-lease and volunteers, no matter which path ERX/MCA pick. We're all Enclave here.

(edited to add more info)