This is a discussion on Would it be possible.... within the Livebearers forums, part of the Freshwater and Tropical Fish category; -->
To keep either a molly or a The guppy, also known as millionfish, is one of the most popular and recognizable freshwater fish in ...
I disagree that a single Guppy cannot be kept in a 2.5 gallon properly equipped and maintained tank, especially if it's planted. It is not the best solution. They are shoaling fish so they do like to be with their own kind.
Probably an entire litter for a month or so before they start needing more space. As far as fishes go, guppies are relatively clean, and that's the concern. You have to make sure the bio load can be handled. Guppies don't zip around like danios, so it isn't so much open water that's the issue. Once the fry get big enough to start taxing the bio filtration in a probably bare-bottom 2.5, though, it's time to move them. When that happens will depend on a lot of variables like how many fry, water temperatures, and so forth.