While a 75 gallon tank might be a fine *temporary* set up for either animal, cramming both in there is likely going to end with the fish dying. I used to have a pair of florida gar, about the same size as what you're describing in a 150 gallon, and I still fill like even that wasn't enough. I also know from experience than my gators have all made quick work of whatever fish in with them, even when they were little. (Tried some adult South American, Red Terror cichlids in with one when he was a month old....and he promptly dispatched them for me.)