AFAIK only the true upside down catfish-synodontis nigriventris and the asian upside down- mystus leucophasis, swim upside down most of the time, however some other synos-nigrata, eupterus etc. do swim quiet alot upside down when juvs. but as adult swim normally!!!
Also, the true upside down catfish only gets @ 3-4", so when someone says they have a larger upside down its most likely nigrata that can reach 8"+!!!!!