The small upside down cat (nigriventis) really prefers to be in a school. Right now I have only one, and I basically NEVER see him since he hides in the driftwood holes. When I had 4 of them, they encouraged each other to come out and play. They are nocturnal, so don't expect to see him much anyway unless you don't provide hiding spaces for him.
I hope to have 10-15 of them together in a large aquarium one day.