If the synodontis aren't what you're looking for, I have kept and really enjoyed Zamora wood cats, cheap and easy, and pretty, and currently have two dwarf giraffe cats. Both species seem to max out about 4 inches. I might be wrong, but I think a lima shovelnose only gets about 10-12 inches, and there are few doradids that don't get huge. A flag tail armored cat (really blanking on species names here sorry) are also pretty cool. Polka dot pictus are great cats- I recommened a school of at least three- they are very active. there are also frequently a variety of glass/sun cats in LFS's- I see them at pet smart alot- they don't get huge, school, and are non-aggressive generally.