I would think it would be ok to have a single apisto in a 5 gallon, but any more than that and they're going to be fighting over territory -- they like to stake our their own space.
I've got a pair in a 6 gallon right now, but I'm preparing to move them into a 20 gallon. And even though it's a pair, and for the most part they get along fine, every once in a while the male will chase the female and she's got really nowhere to go, so she ends up hiding in the back behind the plants. Not really fun for her.
As for tankmates, from what I've read they should do well with other fish (unless they're spawning, and again it's that territorial thing). I've just got a couple of ottos in there with them right now.