I would advise against mixing species from the same genus (not trying to insult your intelligence, but it goes "genus species" for scientific names - so cynotilapia is the genus) because they will usually cross breed. While many don't mind hybrids they usually don't turn out as colorful as you might expect. Sometimes they do, but a lot of times they don't. So, to make it simple I wouldn't mix multiple Cynotilapia groups. In a 90 gallon I would suggest 20-25 total fish. You could put in a few more if you have enough filtration and are willing to keep a solid water change and tank maintenance routine. Also, try to keep species groups with a ratio of 1M to 3-4F. This ratio will keep aggression spread out and give you great breeding behavior which is fun to watch. Also, it keeps a single female from continually getting picked on to breed. So, in a 90gallon I would suggest about 4-5 species (depending on the ones you choose) with about 5-6 of each species. You could start with more since juveniles can be hard to sex, and then cut down the group as you go removing extra males or females to get your final group.
Also, here is a good website that has a pretty comprehensive list of the mbuna available in the hobby. If you go to the beginning of the "Profiles" you can also check out the other types of cichlids from the other rift lakes and other areas of the world (but I wouldn't mix different types). Here is the link <http://www.cichlid-forum.com/profiles/category.php?cat=2>.
One thing I will also mention is it is usually good to try and choose species that have similar levels of aggression. For example it doesn't usually work to keep kenyis (one of the most aggressive) with yellow labs (one of the least aggressive). Once they reach sexual maturity the kenyis will usually continually hassle the yellow labs until they die from one of a number of problems.
Lastly, I would highly recommend you post up what you think you want as a "final" stocklist so people can comment on potential problems. This is especially helpful because some species need to be kept in larger groups (such as demasoni, no less than 12) to keep them from going crazy on other fish.
Good luck choosing, there are just so many great ones out there.
