Boesmanni rainbows, Red rainbows, giant danios, Black Skirt tetras, Scissortail rasboras, Some people even use calm mbuna, like labidochromis caeruelous, and many others.
A festae is a real demon, I would personally try mbuna or leporinus, (the nastiest & toughest available), and I would have chosen a Cichlosoma or Amphilophus rather than a GT. A 120 is pretty small for festae, but who knows? Stranger things have been done successfully. (I have no 'big cichlid' experience)
BTW, keeping your tank a little cooler than it should be will do wonders for keeping down aggression, although it might make your fish less colorful/active, and will almost certainly eliminate any chance you might have of anything spawning.