A lot of mixed opinions on this on this. However the TSN is a eating machine and will eat all of the food leaving none for the gar. So you will have to make extra effort to make sure the gar is getting proper nutrition i.e. eating enough food. I would get the TSN that is smaller than the gar by at least a couple of inches instead of what you have. I recently acquire one that is 18" and he lives with two FL gars. one 18"-17" and one that is 10". Mine was raised with gar so i have had no problems with him. What size tank do you have?