I agree with ImperfectFaerie. One or two guppies might indeed survive in a 2.5 gallon tank, just as a St Bernard might survive in a small bedsit, but surely you want to provide your pets with more than the barest survival. It wouldn't be much of a quality of life- remember you can't take the gups for daily walkies, the tank is all they're going to get. Guppies are quite active fish who love exploring.
A betta is a much better bet- being slow moving creatures they don't mind space being restricted, they can breathe air so are less affected by small surface space, and they can get quite into interacting with their owners.
Other options might involve cherry shrimps, or maybe an African dwarf frog (though make sure you don't get a clawed frog by mistake as they grow much larger).