Hello Jen welcome to Fishlore!
I think that size would be ok for one pair. If you wanted to go with 2 pairs you might want to try a longer tank.
As far as other species i dont think any other africans would be ok in that sized tank... So maybe you should look into other fish like schooling fish. Maybe a school of Blackskirt tetras? Thos are really nice and look awesome in schools
as far as catfish... you could try a rubberlip pleco
Not so sure about corydoras... you might want to check on that with another member.
Enjoy the site!