Skilard,
Ok after extensive research, another maybe better mexican UU posibility.
"Dragones "
The Mexican army had 1 brigade of elite dragoons "Dragones" at the start of the War between México and the United States. In reading, it seems that they did fairly well for a while, routed a few troop movements, while being outnumbered, they fought more efectively than their American counterparts. At least until general Arista (you had him listed as hero - some honestly wrote him up as a traitor here), sent them out into the open in a charge (outnumbered drastically) to get blown to bits by American "flying infantry"(called flying because each member of artillery team had a horse to manuever the cannons into position quickly).
so in short maybe "Dragones". México only had one brigade though.
Mexico was really in horrible military shape after loosing over 1 million people in the Revolutionary war, all the uprisings, and the chaste wars (probably 200,000 more).
If you don´t use Juarez (now my first choice as letterhead) and use Maximiliano instead just use some French unit but you have to change the whole civ then as it wouldn´t represent México.