I've got a GT with 2 oscars and for the most part they leave each other alone... The GT only chases after my flowerhorn and severum. The 2 oscars on the other hand seem to bother each other. There usually will see a small chase where my larger of the oscars chase the GT when he goes into frenzy mode chasing the other fishes.
Other than that when they were smaller they only got into chasing each other the first week I got them.