You cant go with Africans, They do the territory and aggression thing a bit different then the SA's(they would not be as happy or healthy as they could). I have had Oscars, Fire-mouths and convicts with great success in the past. But with any Cichlid you may have trouble in the future. Anything besides a female convict is future for disaster. You only have 75G to work with. Mine (two each) where in a 150G tank which is much different. Stick with the SA cichlids for sure and get them all about the same time and size. If you do an Oscar remember they grow SUPER fast and BIG. If you go one Oscar you can only go with a couple convicts. The texas Cichlid may be an option to. I am sure others have experience with this set-up as well and will lend some good or even better advice for you.
are you thinking mumba in lieu of the convicts? If you want species variety I would go mumba.
Convicts get nasty when they breed and they breed like crazy.
Sevrum are a nice fish to mix with them. Texas and Green Terrors get a bit big and mean for convicts, Oscars are really a fish that should be kept alone. Honestly if you really like convicts i would go with various color varieties of them and call it a day personally.
I might consider one severum, ONE convict,one Blue acara,and one firemouth for 75 gallon tank.
As has been mentioned, convict's are nasty mean when and if they should spawn and in my area,, you can't give these fish away.