Most people say they really can be aggressive as adults, and sizes vary.
I would like some fish be kept as a single or pair, and i saw my old teachers GT and it was STUNNING
no, i'm not saying GT are punks, I meant Pink Kissers.
Green Terrors CAN be aggressive or fairly communicable depending on how they were raised, tankmates, tank size, and sex.
Females tend to be a lot worse unless you have a male who is the dominant fish in the tank, but again it boils down to "Is the dominant male GT dominant without question or does he have challengers that he KNOWS may pose a problem in the future".
Extra aggression can be seen in dominant males when kept with Parachromis, Amphilophus females, as well as FH females or even pushy JDs and FMs or Convicts. A male GT isn't necessarily a savage killer but it is like a Pitbull and has to win at everything unless it gets beat within an inch of it's life by a tankmates.
Then it may submit to a more aggressive fish, for instance I have a male GT who has been "arguing" over dominance w/ a male RTM for the past 6 months, and I have a feeling the GT will need to be moved or the Red Tiger will move him for me, if you catch my drift.