Don't use yellow onions in your Mexican food, unless you're going for a bland flavor; they won't give you the authentic bite you're looking for, and tend to sweeten too much when cooked. Use white onions. They work best in salsas as well as in cooked recipes.
Despite common belief, you can enjoy a good wine with Mexican food. While you should drink whatever you like with whatever you like to eat, good combinations include a zinfandel for chili, a German Riesling or Italian Chianti for cheesy dishes, or Portuguese red wines with paella.