I wouldn't add another cichlid, but if the aggression isn't that bad I wouldn't worry either. My eight-inch polleni chases my six-inch Red Bahia some in a 55, where they're the only tank mates. It's not that frequent though and both fish are healthy with no marks or scars. Eventually, they might not be able to withstand each other, but that's not going to happen over night. If you have that GT in there alone it's going to be a pretty boring tank for a few years, until the GT does reach a larger size, and if it's a female it's not going to get THAT big anyway. Dithers would be good, just make sure they're big enough not to be nipped to death.