In reference to tank size I think it is important to remember to keep sight of what things will be like once the fish are grown. Yes a 75 gallon should be enough to house a young devil and terror for some time. Once the fish start getting big the aggression is more likely to increase. To me another important point to remember is what commitment you will have for your water changes. Two large fish in a cramped tank is more work. More water changes and more filter maintenance. When keeping fish it’s easy to get excited and want to add more when they are small. For me I try to think about how much effort (time money) I can give once the fish is full grown in its tank. Most of its life will be when the fish are big.
