depends on how you set the tank up. Id stay away from pictus. They prefer groups. and while 6 pictus looks really cool. 24" isnt a heck of a lot of swimming space.
Cories are a good suggestion.
Synodontis nigreventis are ok. but for a decent group i think a 29 would be better.
Bumblebee cats. M. iheringi are awesome.
one maybe two spotted raphaels depending on the stock and filtration
Any numerous of midget pl*cos it would take me awhile to type them out.
Theres a couple of other cats, but they escape me and i dont want to get my books right now.
My apologies if I repeat any already mentioned. Then again, repeats suggest that those fish might be a really good idea
-South American bumblebees
-Banjo cats
-Synodontis petricola (one of the smallest Syno species, I believe. I would hesitate to introduce any of the more common species to a 20g, as they can get a bit larger than 6")
-Woodcat species, such as C. perugiae or T. intermedia
-Spotted raphael cats - tend to stay a good bit smaller than their striped cousins.
If you want to extend your general catfish inquiry to include plecos, that will open you to an even wider range of possibilities.