I'm not too experienced with catfish, so others may need to jump in here, but what I can tell you is
Corydoras catfish are also shoaling species, so again need to be in groups of at least 6. Pygmy cories are fairly small (the clue's in the name there!) and there are some other smaller ones like pandas that may be ok - like I say, my knowledge ain't too great.
With Plec Catfish, you have to be REALLY careful that you know what you're buying, as some will get massive. There is a list, as a stickied topic, somewhere on the forum, probably the Plec section, that lists those species that don't get above 8". You want something that won't grow above 4-5", and one only. From what I've read, they produce waste like there's no tomorrow, although I think Tizer has a Bulldog Plec, which he says doesn't produce too much waste. Maybe worth a PM to him to check.
The other option is a couple of Hillstream Loaches, which are also a suckermouth species. They stay very small, BUT you need to be aware that they really need excellent oxygen levels, fast flowing water, and low temperature, as this is what they get in their natural habitat. I keep mine (
Ctenocephallus Gastromyzin) in water that is really too warm for them (I did the usual thing of not researching properly before buying, but that was a while ago).
