The Synodontis eupterus would probably be ok, they are quite adaptable regarding water chemistry and can normally look after themselves in most community tanks.
However, Synodontis nigriventris would be an awful idea in a Rift Valley community. These synos are timid in character and eating habits compared to most synos, which is why they are great for "African oddball" tanks with fish like Butterfly Fish and Leopard Bushfish. But in addition, they are extremely social for synos, I have 11. I cannot see mixing these with Mbuna ending well, if they don't die through aggressive injuries, stress induced disease will hit them hard.
The real answer here is look to the Rift Lakes themselves. Granted, there is just one Synodontis species that lives in Lake Malawi (S. njasse), but there are a few more options from Lake Tanganyika (luccipinnis; multipunctatus; grandiops; petricola etc.). But be warned, you may not get many cichlid fry depending upon your choice, these catfish will try and mix their own fertilised eggs with the mouthbrooding cichlids' ones, they develop quicker and eat the cichlid eggs/fry.
Alright. Thanks for the information. I really love my S. nigriventis and don't want him getting hurt so I guess I will have to see if the LFS will take him. I actually got him as an unplanned purchase because they had put it in with some more aggressive syno species at Walmart and they had almost completely eaten his fins off and he was under an inch long. I bought him in attempts to save him and that was 2 years ago and he's doing great.