Well, most catfish are too big to put in a 5 gal. You could fit a few cories, but the problem with that is cories can be kinda sensitive (you really have to watch a 5 gal to make sure it stays stable) and they're happier when you have them in groups of 4 or 5.
I mean, you could put 2 emerald (or albino) cories or 3 panda cories in there, but most ppl would tell you not to. You could get 5 dwarf cories, but they're so small it's probably not what you're looking for.
Really, that's kind of the way it goes with a 5. gal. A lot of the stuff that you can reasonably fit in there is small stuff that is kind of sensitive and/or likes to school, and you can't fit a good sized school.
If you can keep everything stable, ottos would be really good bottom feeders for a 5 gal.