Green Terrors are not that stupid,they can figure out who they can touch and who they can't touch at all.I own a 4 inch Green Terror, that goes after only certain fishes in my tank,other then that the GT is fine with everyone else.As for having parrot cichlids,they don't really have big mouths at all,so it maybe risky that they can't defend themselves that much.Its a hit or a miss with any fish you put into the tank,also I hope your fish tank is at least 55 gallons or above to do this sort of thing.If you want a GT that wont grow as large like the males do,look for green markings by the face that will tell you its a female,while the male has blue markings.