with upside-down cats (UDC), be careful because many Synodontis are labeled as such. I have two Synodontis nigriventris, "true" or black belly UDC. the family (Mochokidae) sometimes also called "squeakers" but have yet to hear them make sounds myself.
they spend the majority of the time inverted, even dashing around the tank that way when they get startled. you can tell them apart from the others because they have reverse counter-shading: they are darker on the belly and lighter on the back. their camouflage is backwards from other fish, to reflect the fact that they are upside-down more often than not.
they're fun, though hard to see because they hide until well after lights-out. they get to a max size of 4". they need really good hiding spots/caves. my large male is so fond of his cave that once i took it out to clean it under tap water during a water change. i left it out for nearly an hour before putting it back in, and he came sprinting out! he had been crammed in there the whole time, reluctant to leave for any reason. he was fine, tho, and is still going strong. nice, fat, round belly.
they, along with most Synodontis, will eat just about everything! I supplement with slow-dissolving bottom feeder wafers so that my loaches can't get everything before they come out. i think the male is eating more than the female, because he's about twice her size! he's about 3.5", and she's barely 2". males are usually darker than females, and the females have a stouter, rounder body.
i've heard conflicting things about whether S. nigriventris will get along with cichlids. this species might be a bit small for Oscars and such, so another larger Synodontis might work better. however, like i said before, many species in that genus are also referred to as various UDC's, so they will also swim on their backs some of the time. anyway, they're all hardy, not picky about what they eat, peaceful, and fun!