I have kept several in a 29 gallon with a good bit of rockwork without any problems. It would be best to add them at the same time though. Mine seemed to pretty much ignore each other. One of my fav little fish.
I kept two of the same species in a 125 gallon that fought non-stop until I had to remove one. You'd think it'd be enough space for such a tiny little fish but there ya go. YMMV.