PJ cardinals do fine individually and I've got one in a 20gal, so I'm sure a PJ would do fine in yours. I'm not sure how social I would call them; M/F pairs are supposed to be fine, but they are darned near impossible to sex when young (I'm not convinced it's ever possible to do with high accuracy). Ending up with two males by accident didn't go to well for me when I tried it in my 55gal, so I had to move one out eventually. Some say that it goes better with larger groups, but I certainly wouldn't try larger groups in a smaller tank. So, in smaller tanks it's probably better to stick to one unless you find a pair with some kind of assurance about the genders.