I've never had trouble taking money out of machines in Mexico (or anywhere else in the world where I could find a machine actually) with my Visa debit card. I'm sure ATMs will be plentiful in SLP.

It's probably worth bringing a couple hundred in US$ just in case you travel out of the city or to tide you over until you can get things straightened out if for some reason the card doesn't work. And if you don't have to use it, it's no problem to take home with you.