I have never had any success keeping different types of Cichlids in a 75 gallon, or similar size, tank.
Once they get to adulthood, someone is going to die. Many people will tell you otherwise but I doubt they had full adults. I kept a Flowerhorn, Black Belt, two Red Devils, Buttikoferi, two Convicts, and a Green Terror in a 55 when they were 2-4”, as time went by the aggression was just insane at once they hit 4” everyone had to be moved into different tanks. I left Black Belt and a Red Devil, they were fine until they reached around 6-7” and then had to be separated. Once Red Devil hit around 9” nothing could go near the tank without him flipping out.
Anyway, moral of the story is, he will only be able to keep one Green Terror, or a Green Terror pair. Keeping it with another fish(s) may work for sometime but it will simply be impossible once the fish will start growing up.