If you like corys consider the Hognose Brochis (brochis multiradiatus) these are a shoaling species and grow large and chunky enough to avoid being food for a Severum, i have kept them with Jack Dempseys and Severums in the past no problems
as Suggested Hoplo Catfish Hoplo (megalechis thoracata) very active and always on the go ,can become quite tame, do well in groups or alone.
These are just two members of the Callichthyidae family you also have Pimelodus family among many others from SA .
One of my favs is Ornate Pim's (pimelodus ornatus) which do well in groups active thou very boisterious around feeding times and a bit skittish , but would do well i a 300 gallon tank.
You have plenty of options with SA catfish