Do you plan on keeping the current stock? I only ask because none of them are south American cichlids, and you state you want a SA cichlid tank. And honestly, a 75 gallon with a BP, pair of convicts, and a rainbow shark, is just about stocked. The pair of convicts, if breeding, will probably take up half the tank. BP's get pretty large as well. Te hconvicts are CA cichlids and are more aggressive than most of the SA's. The BP is a hybrid fish and can also be quite mean. You might be able to shoehorn some other non-cichlid in there. But I would definitely not be adding any relatively peaceful SA cichlids to this tank.
In my opinion, your best bet would be to find a home for your other fish, if your goal for this tank is SA cichlids. You could do a nice little community of smaller, chilled-out SA cichlids in there, but would have to get rid of the CA guys first, or they would probably get torn to shreds.
Also, should post this in the cichlids section, this is the salt forum