Laurie, I think it's fine to monkey with it and see what comes up, but when you query I think you'll need a narrative arc. My agent made me rework a NF PB a couple of times last fall to strengthen the story thread so it wouldn't be slice of life, and it still didn't get picked up. I learned a lot doing that book but I also spent a lot of time on it, and a paycheck would have been nice.
Last week my agent sent me back a fiction PB ms saying it's a delightful concept book, but doesn't have enough of a straight-up story--it's too slice-of-lifey and they're not selling. So I would definitely address that before you query.
I don't have much expertise on the second question, but I would guess that books that deal with Mexico would be very welcome.