I kept and bred many GT. A male GT can reach 10", and female about 8". IME, GT is one of the least aggressive big cichlids, and I have kept GT peacefully with smaller African peacocks, Tangs, and CAs including an angel fish.
I have never kept Festas. But from what I read, Festas male can reach 15", and female 12", so there is no way GT can dominate over RT in the long run. I don't know if RT necessarily grow slower than GT or can they cross breed as I have never kept both fish together. Given that GT and RT came from similar habitats in the Pacific rim of Ecuador and Peru, they are likely geneically closed enough to rule out that the can never cross breed.
Wish you can get replies from people who have actual first hand experience instead of giving you opinion based on speculation.