Syno. eupterus is not a good choice for a 40-50g tank as they can get 8-9". A better choice for an african tnak would be a couple syno. multipunctatus or syno.petricola as they come from the rift lakes and can take the higher ph. PLus they are smaller, only getting about 4-6". But you have to get at least 3 of the same species for them to be happy.
I don't know what you mean by "good." My synodontis catfish are very peaceful and they clean up all the food my Africans miss. They aren't out a lot during the day, they usually come out at night.
if they are smaller cichlids i think you could also try synodontis nigriventis.... upside down cats. they school but are smaller, only around 3 or 4 inches full grown i think.
fascinating little fish, good hiders. they should be good with the ph if acclimated slowly. i used to keep a few and they even survived breeding cons and the mbuna i used to keep.
I agree for the most part with Valerie; I think a Syn. eupterus is pushing it in a 50. For the cichlids you note, a quartet (+/- 2) of Syn. petricola would be ideal. Kept in groups, they're as active as any synodontis and very comical. I've got both eupteruses and petricola and I'd pick the petricolas hands down for a tank like you describe.
Syn. multipunctatus are OK, too. They look a lot like petricola, except a bit bigger. They aren't as active as petricola, IME.