African Butterfly Fish (upto 5)
Leopard Bushfish (somewhere between 4 and 8)
Congo Tetra (good sized group of ~15, two females per male)
Upside Down Catfish (nice big group, say 15)
Reedfish (5 or 6)
African Brown Knife Fish (1, I'm not aware of them being social, but could be wrong)
Distichodus affinis (1, far smaller than sexfasciatus or lusosso, literally just read this one can be social with conspecifics)
Synodontis notata (1, my 25+cm specimen is a gentle giant that cruises the lower tank levels throughout the day) and/or a group of 5+ Synodontis schoutedeni (infamous for forming "golf ball" shaped stomachs!)
I'd love to say a big group (15-20) of the schooling midwater Pareutropius debauwi catfish, that grow a little bigger than the more common P. buffei, but I'm not sure if they really do reach 10cm in hobby tanks and whether they will be safe enough from the Reedfish and Bushfish in this setup.
There's also a range of Birchirs, but I don't know enough about them to say how they would combine with the above list.