Synos..
Finally something I can chirp about.. I have and have had several different species of Synos which we have established is the catfish of choice for ANY african cichlid tank... I am no expert so treat this as such.. disclaimer over..
Remember that what you keep matters too. Mbuna fit for the same spots to hide in, Haps not so much.
In order- Petricolas (Lucipnnis) are ok and contrary to popular opinion can get pretty decent in size in like 40 years.. (ok maybe less than that) I have seen them as big as 3-4 inches+. I was shocked. Wouldn't be my first choice though. Excellent for small tanks though with small fish.
Eupterus- super cool, sail fin, and grow. I have 2 right now that are 6-7 inches long at least and a couple inches wide and tall (not including the 2 inches of sail fin. They grow pretty quick and will probably not outgrow your 75 but may outgrow your cichlids. Sounds silly, but a large Syno Eupterus will get as big or bigger than Mbuna, and most of the more popular Haps.
Syno Multipunctatus are wicked. I currently kinda, sorta breed them and once they reach the point where nothing in the tank can eat them, you throw 6-12 in there. They school, have awesome color, more active than most Synos, and hold their own. Not to mention they might breed for you.
Decorus on the other hand get good size, BUT they have always been very shy I have found and can get pretty big.. I have also found that regardless of size they are not as hardy as my other Synos. They seem to be the canaries in any tank. If water parameters become off, they seem to struggle the most. Just what I have found..
Syno Ocellifer is very much like the Eupterus, but no sail fin a little more elongated and different color/spot pattern. Great hardy, bigger Syno too..
Hope this helps!
-marc and family