African cichlids need a bit of over crowding to keep down the aggression. Extra filtration is must. Choose species of the same aggression level and size roughly.
We have 26 fish in our 120g and the mix is fine. You can keep about 15 fish plus a pleco in there. Sex ratio 1 male 2-4 females.
Highly aggressive species are usually heart breakers...so pretty, but so evil. I recommend the less mild species. Oh and add 4-5 fish at a time...one species at a time. Never add two of the same species...it will lead to disaster.
Stay with species from the same Lake unless you choose to do an all male tank.
Lake Malawi cichlids are the most aggressive African cichlids I have seen compared to the rest.
Here's an article which gives an idea of what can go together & what not...
http://cichlidforum.com/articles/cookie_cutter_75g.php