I have one in my 10g...had 2 but the other died. And he has been living happily for a couple months. He still hangs out on the glass. I've been meaning to get another, but haven't gotten around to it.
My 3 in the 29g do hang out alot. They aren't always together though.
I don't think they are schoolers...more like shoalers (like cories). They don't necessarily swim around together (in a school), but they do enjoy the company of their own.
I would add another. They don't add much to the bio-load, so go for it. Feed them some algae wafers...mine love them! They even eat shrimp pellets every now and then.