I've currently got a 15 gallon tank set up with some live plants, 2 ghost shrimp and 1 bamboo shrimp.
As far as fish go, I have two serpae tetras, and two phantom tetras. The employee at my LFS assured me that they'd school together because they're similar looking. Probably.
They don't fight or anything, and they seem to get along well, but I've never seen what schooling fish are supposed to look like. I'm content to leave them as is, if I wasn't going to make any other changes to the tank, but I'd like to go ahead and add one of my bettas, in the hopes that he'll enjoy the extra space, and hunt my thousands of nuisance snails.
Do you think it's safe to add him, or should I go ahead and get more tetras so they feel more secure? And how many do you think I can add before I'm in danger of overstocking the tank? I believe my hob filter is rated for a 20 gallon, but if necessary I can rig up an extra sponge filter for bio filtration.