Lima is a good choice, could have more than one too. If you want a bit of action from a medium sized good looking catfish, see if you can get hold of a "bolt catfish" , aguarunichthys torosus, make great pets, can be hand fed and are active, especially if you put a powerhead front to back with air intake, they love it.
Think you need to look at the wimple piranha idea, from the little i know of them they survive by eating fins and scales off other fish - I could be wrong but I think deadliestviper7 is talking about leaving out the lima not keeping them all together.