Don't JDs and GTs get to be about the same size? The GTs I've owned have been just as aggressive as any JD I've had so aggression could still be an issue.
maybe im wrong but it seems to me its easier to grow a larger jack... ive seen 11"-12" JD's and never seen a 12" GT
... and as for agression yes they are both about the same when it comes to cichlid agro but they are worried about their bottom feeders not other cichlids
ive had GT that were agressive but never to bottom feeders or anything similar and owned a 10" Jd that destroyed and ate a 6"+ shark leaving nothing but a red fin...these are just my experiences.
I wouldnt mix the GT with any others in the 55g im saying keep just one GT or possibly a pair.