One of the smaller "long whiskered cats" that is most commonly imported is p. pictus or pim-pictus or simply pictus cat. They get about four and a half inches but they are very active swimmers and need a tank atleast four feat long to be really happy (standard fifty five works great). If you plan to move up to "semi aggressive" fish you'll need a tank thats a little bigger anways, why not get the fifty five set it up get four pictus (must be in groups) and then your aggressive fish, i dont suggest oscars with pictus and a fifty five is kinda small anyways. I've had good success with geophagus cichlids,rams,larger tetras,eels (will outgrow a fifty five),banjo cats,callichthys cats (related to hoplo but bigger),p. maccus,rusty plecos,gibbis,dojo loaches,guaramis,african butterfly fish, i guess pretty much anything they cant eat (their mouths are bigger than you'd think) and anything that wont eat them or bother them. I'm trying to think of some "semi aggressive" fish i've kept with them and the only things i can remember were fish that were only with them until they grew big enough for other tanks. the eels would be a good tank mate if you had a ninety+ gallon tank. Good luck, sorry for babbleing