Well, they aren't Brachyrhaphis hartwegi; like all the other species in that genus they should have the black and orange bands in the dorsal fin, and bars on the body rather than spots. So let's play ichthyologist.
Other common genera from the region are Gambusia, Poecilia, and Poeciliopsis. The length of the gonopodium is too long for Poecilia and too short for Poeciliopsis, but about right for Gambusia, which is closely related to Brachyrhaphis. By dumb luck, I started at the back end of that genus in Freshwater Fishes of Mexico, and found G. yucatana, who's range includes Lago Peten in Guatemala. A quick look at the photos of preserved specimens verified the body shape (a bit deep for most Gambusia).
So, I would say the mystery fish is most likely Gambusia yucatana, not currently in the hobby SFAIK. Until now, of course!