Expecting every fish of a species to reach the maximum potential size is naive, folks. It's akin to sayin' I should be 7' 2", weigh 350 lbs. and have had a stellar, hall of fame career in the NBA simply 'cuz I'm human. A certain percentage of any species will get larger than average, a certain percentage will remain smaller than average, and the majority are gonna hover in the average range. Neither GTs or JDs have an average length anywhere near 12"; 8" for a big GT male is far, far away more typical than 12", and 8"-10" is more typical for JD males, with females smaller in both species. That should be the basis for answers, not the potential growth of that 1 in 1000, genetically superior individual who sets the max size for the species.
A 55 gallon is more than adequate for average members of either species. Just like with any cichlid, mixing species should be done with caution. Some individuals are mellow and some are more aggressive. Yer best bet if ya wanna try an individual of each in the tank is to buy 'em young and introduce them together. Rather than a common pleco, look into some of the mid-size species, such as the larger Peckoltia sp. With adequate filtration and a weekly water change of 30-50%, you should be able to maintain the tank as long as one or the other does not become overly aggressive.
WYite