You want to do your cave training in Mexico, but not in sidemount? The issue isn't just getting through small holes, but all the stalactites, twinsets will limit you*. Honestly, I'd suggest cave training with the likes of Steve Martin, Steve Bogaerts, Natalie Gibb, and others that people here have recommended. You just have a glut of awesome sidemount cave instructors down there. It seems to me be an easy decision.
* I know a lot of twinset divers are likely to react to this, but I have a number of friends who are cave diver instructors in Mexico, and that is the consensus. Not looking to start bickering with anyone.
Didn't realize you got cave certified, did you do it in MX in SM?