Flowerhorns are out of the question as they are very aggressive fish and shouldn't be kept with others unless in 200 or more gallons. If you're willing to go severum only, you can keep 2-3 in a 75 gallon. Once you start mixing species, you'll need to have at least 100 gallons for two fish, although a female Dempsey and a female severum might work in a 75 since they are both only about 6" fully grown.
With cichlids though nothing is ever sure. You might get a 6" female Dempsey that is vicious enough to take over an entire 100 gallon tank. Or you might get a 10" male texas and an 8" male severum happily co-existing in a 75 gallon. It is really down to finding the right fish and observing very carefully, being aware of their behaviors and knowing when to intervene.
You say that this is going to be in the very distant future. If you have time and a 10-20 gallon tank, try housing some dwarf cichlids like rams and kribensis. It'll give you a good beginner education on cichlid behavior