I've never done it, but I think you can trade alot of different currencies at those types of locations. But you'd probably get better rate if you used USD to pesos instead of sterling to pesos. I remember like 10 years ago I one time asked if I could change Swedish crowns to Pesos, and yeah but I would only get 2 pesos for each SEK, when at that time (and still is around that rate) the official rate was 5 to 1. Sooo horrible rate, just because it's not a valued currency by anyone here.
Yes, you can. A few years ago, I had some sterling pounds left from my U.K. trip. Can't remember if I exchanged them at Banco Santa Cruz or Popular, but I do remember that no U.S. dollars were involved.