I kept a JD with a GT for a while, and everything was fine until they were both about 6" . . . then it got ugly, with the GT getting the worst of it . . . no actual damage was done, but the GT started to spend more and more time just hiding, not coming out to swim or feed or anything.
he was a beauty, too . . . which was good, because I got decent $$$ when I traded him in at my LFS
this was in a 90-gallon tank, and other than a male convict they were the only cichlids in the tank . . .