My GT is about 5 1/2 inches and as stated above he makes a great tank boss. I have two Oscars, a Jack Dempsey, a Red Bay Snook, and a pleco in my 180 with him. The GT is extremely territorial with the Oscars and Jack Dempsey, but almost completely ignores the smaller Snook. While extremely territorial, the GT mostly just threatens without doing any real damage. Only when the MUCH larger Oscars don't back down will the GT really attack, and the removal of a few scales is usually enough for the Oscars.
Its fun to watch the Green Terror flare out its gills, and chase away Oscars almost twice his size. The Oscars could easily tear him a new ***hole, but he puts on a show and the Oscars are intimidated.