they both will grow quickly, the flowerhorn perhaps a bit more rapidly. Be wary, as they mature FH's often need to be kept alone as they have VERY aggressive personalities. But if you have a large enough tank it may not be a problem, esp since its not another FH.
as for the nuchal hump question, I believe you will notice a change at around four-six months depending on water conditions and other envioromental factors. Also, males will develop a much larger hump than females- (i am not even sure if female FH or GT will develop one!!) Hope that helps a little bit, and have fun with your fish! Good luck...
btw, what size and how much did you pay for your flowerhorn? i paid five bucks for an inch long fry here in seattle, but the same store had four inchers for $95.00, and full grown adults for 500+! crazy prices but great fish!!!