I would suggest adding anymore fish to a 75G if you have a blue acara, severum, choc. cichlid, and a firemouth. Not only is that combo not very good and you'll have very bad aggrestion in the tank, most of those fish will hit 10in. Which they will need a lot more swimming room then other cichlids.
If you wanted to do an all african cichlid tank, I would say to put 10-11 african cichlids in a 75G. Slightly overstocking cichlid tanks cuts aggrestion down a lot, do to the fact that the cichlids won't bother getting much territory since they would have to be watching it all the time because of all the fish.