And if you want a real hoot, keep the Giant Inverting Catfish, a cousin of the small ones, known as Brachysynodontis batensoda. Gets to 8-10", very dark, high dorsal fin with extended filaments, and spends most of its time upside down. Largely nocturnal, but harmless except to close kin, it won't even eat guppies. It may spook other non-noctural fish by crusing around the tank in the dark.
The standard upside down cats do best in groups - they are much less bashful that way as they feel secure with kin in sight, much like Cory cats do, and while young will school a bit but not as much a Cory cats.