M60_to_LGA

Then you're fine. Don't overthink things. Assuming you're only there as a tourist for a week or two, I'd take maybe $200-300 US in $20s, just to use as emergency backup in case the entire ATM network collapses or you find yourself in the middle of a remote village where you stumble across some beautiful handicraft you just can't live without, the seller doesn't take cards, and the closest bank is far away.

If you're going to Cancún and sticking to tourist places, you can probably function almost entirely with plastic. But I'd still take out at least a couple of hundred dollars' worth of pesos to pay for snacks and drinks, public transportation, souvenirs, entry to museums or tourist attractions, and the like. Also, cheap restaurants like taquerías will often be cash-only.

EDIT: Not that you asked, and you may already do this anyway, but I'd suggest calling Schwab to notify them of your travel before your trip. I never used to do this, and then I had a couple of instances where I tried to use my debit card to get cash in Mexico, or even Canada, and they froze my card due to suspected fraud. Since then, I've made a practice of submitting a travel notification beforehand, and I've never had a problem.