What is compatible with a 5'' Green Terror in a 20 gallon tank?

Nothing because you will need a bigger tank later. However, you will not need a 75 gallon tank. Although they can grow to a foot, this is rare, and 6-10 inches is far more likely. A 40 gallon tank should be able to accommodate in the long run. A thirty gallon tank might work, but a 40 is better especially if if you want tank mates and if it grows larger than 8 inches. As for tank mates that would work in a 40 gallon tank, some types of catfish, rainbow fish, and barbs would work.

Hope this helps!!

Nothing, you need to focus on getting him into a much larger tank instead of cramming more fish in there.

At your fish's size, he should be in at least 40-gallons. Eventually he'll need a 75+.

If you want friends for him, you'll need at least 100-gallons.