having done this before in a variety of combinations (as have some others on site), i would highly suggest against putting the two together. the gator gars will quickly surpass the size of the Florida, and are very aggressive fish (most aggressive of the gars as well). they will end up beating on the Florida gar and you may deal with anything from major fin shredding to the gator gar actually trying to eat the Florida.
if these were sub-adult fishes there might be a different scenario/possibility, but with young of the year (YOY) fishes, i would highly suggest against it. best to stick with the Florida gar, you can keep it for a much longer time and also keep it with tankmates...the gator gar is not as compatible--
--solomon