What type of Africans will you be keeping?
I would not put Cories with Africans....they just wouldn't stand a chance with most of them.
If you are going with a Mbuna or Malawi tank, then I would suggest no scavengers as these fish are really aggressive and while some people have good luck keeping Bristlenose plecs and Synodontis catfish with them, you never know if your Malawians will torture them or not. If you choose peaceful Mbuna or Peacocks, then they would have more of a chance of thriving (and should be fine). It all depends on your fish choices though. When I had a Malawi tank, I chose not to have bottom feeders/scavengers because of the aggression. A tank is fine without bottom feeders.
If you are going with Tanganyikans (a better choice IMO
), then I suggest some Synodontis catfish, as these are actually from Lake Tang. Syndontis petricola are the smallest ones and pretty common. S. multipunctatus are the next up I believe and get around 6 inches. I wouldn't put these in anything less than a 40g (might even be better in 50+ gallons). The petricolas should be fine in anything 30g+ since they only get about 4 inches. Your Upside Down Catfish is a Syno (you probably knew that) and would appreciate more friends. If you aren't going to have aggressive Africans, then the USD cats should
be fine....but again, it depends on what fish you have. Synos can definitely thrive in the high pH, but they aren't aggressive at all, so you have to choose tankmates accordingly.
Have you decided on a tank size yet? If you are undecided on your fish choices, tank size may limit you/determine what you can keep.