channel or red tailed cats ..... Just joking mouse faced catfish , dwarf mystus catfish , liver catfish or collared cat fish are gr8 (liver cat 15 cm ) (collared cat 10cm) (dwarf mystus 8cm)
A couple that immediately spring to mind are Microsynodontis like sp.1 (would need a group, they are social like Corydoras), or the equally social Hyalobagrus flavus. In such a small tank, both these suggestions would need to be species only tanks.