My opinion, having kept and/or bred Cyphotilapia for over 25 years, mainly C. gibberosa kapampa (I started with F1 from the original 1990 kapampa imports), and having been a mod/admin for years on a large, and at one time very active, forum dedicated to them--
I generally agree with most of the comments above with a couple of qualifications. First is that for me (and quite a few others) growth in one year from about an inch or so was generally to 6+ inches for most males, by which time, especially with experience, you can generally distinguish the largest in a group as males and the smaller ones as females, though there can be some in-betweeners you're not sure about yet. Ultimately males are larger and have longer fin extensions, usually pretty evident by year 2-3.
Not that it was suggested, exactly, but for the record I wouldn't keep Cyphotilapia with tropheus, virtually anyone on the forum who tried it regretted it due to both dietary and temperament issues, and they often recommended against it-- trophs are quite nippy and aggressive at the same time they're susceptible to bloat when everything isn't right for them. Preferred temperatures are also a bit different, with Cyphotilapia cooler, though there's a middle ground in that respect. It might be possible in a huge enough tank, though I don't know what that threshold would be.
Clown loaches-- a number of people tried this, most of them didn't stick with it, one reason was that at first they were okay, but eventually were too rambunctious for the Cyphotilapia and tended to compete with them rather than complement them.
Some less common, very nice, and geographically correct alternative tank mates for Cyphotilapia worth looking into would be the "feather fins," including Cyathopharynx and Ophthalmotilapia (some of the Cyathopharynx are gorgeous in breeding color), and Benthocrhomis, some call them giant feather fins, though they're not closely related. Just my opinion, but in a tank your size I would keep a few less Cyphotilapia, perhaps 12-15-- which, when you're talking about the Congo coast gibberosa, would make a beautiful display-- and make room for a colony of featherfins.