Just thought I'd toss in my few cents worth since I own one and all...
Green Terrors are not as mean as you would like to believe, however they do choose hold the upper hand in the tank. I think that a lot of their adult behavior depends on their upbringing. If you keep and raise them with aggressive fish, then I do believe that they will become overly aggressive as well. But if you keep them in a peaceful community setting, then you will end up with a more peaceful fish.
Now, let's not confuse nature vs. nuture. Green terrors do carry instinctual behaviors such as territoriality and their defensiveness of it, along with their eating habits. No matter how hard you try to raise a peaceful GT, it's going to eat your neons if he can open his mouth wide enough. Adult GT's (at about 4") need space, about 25 gallons or so to themselves. I would recommend at least a 55g, but a 75g would be ideal if you wanted to keep some other fish with the GT. I contribute my success to having a large aquarium with lots of hiding places--and since I mentioned hiding, GT's absolutely must have a place to hide. They are very precarious, and having a place to run to when they hear footsteps allows them to feel comfortable. When he sees it's only you, your GT will be front and center, waiting to be fed (make sure to feed a variety of foods--flakes, pellets, frozen bloodworms & beefheart).
As for tankmates, I would say most anything will do, but larger or faster fish are a better choice. My gourami does exceptionally well with all of my cichlids; he used to be a mean little fart, but he swims with care now
However, beware of the fish you add after your GT has reached maturity. GT's are very cautious of their surroundings and will inspect every fish in the tank. If you add a small, weak fish, the GT will beat it to death. But if you add a fish of substantial size and "worth," then you should be ok.
GT's are excellent fish to keep--their behavior is one of a kind and quite personable. Heck, mine likes to watch tv with me! He will park himself in the corner closest to the tv and tilt himself to watch. Treat them well and you will have a friend for many years to come.