They are gorgeous little fish. I guess it depends on how often you want to do water changes. You can do a 2.5g with some micro swords and other really small plants, maybe even a java fern, with some cherry reds and a couple snails. I mean, a 5g would be easier and you could definitely put some fish in there; a 2.5g is pretty small, but can be done. I think these pico/nano tanks are great; they're really unique and super cool to put together. I'm thinking you would have to change out a gallon of water every 3 days or so; the plants would help a lot in keeping the water clean. I say go for the 2.5g. If it just isn't working out you can always switch it over to a 5g instead.

__________________
When in doubt, do a water change.