Join Date: Oct 2010
Location: Victoria, Australia
No catfish can go into anything smaller than 10 gallons. Otocinclus and corydoras are active schooling fish that require a decent sized footprint to move around in.
You could possibly try a school of smaller sized corydoras (Corydoras pygmaeus comes to mind) in the 10 gallon, but that's about it. Otocinclus need a constant supply of algae, which a new, small tank does not have, and can be difficult to transition onto other foods.
Clown plecos stay small but they are expensive I believe and like all plecos, big waste producers for their size.
Other than that there are really no other options I could see that would work in the set-ups you have described.
Just read clown plecos reach around 5 inches so I reckon they would do better in a 15-20 gallon tank.