Many people keep synodontis species with their Africans. I have a synodontis nigrita (Lace Cat or False Upside Down Cat) and a synodontis eupterus(Featherfin Cat).
I would recommend the Lace Cat though, because Featherfins can get quite large(quite fast too, you should see mine!). There are other syno cats that stay small, I just don't know their names off the top of my head.