Your best bet is to go with a nice florida gar to start out with. Your tank size is going to dictate how often you need to upgrade. If you start out with a small gar ~4-6" then I would start out with a 20 gal tank because most likely it's going to only eat live food at this stage. The smaller tank will help the gar find food easier, granted it will find food in a larger tank but if you are planning on only growing out the gar, the tank will look very empty. It's all going to come down to your space constraints, if you can't get a larger tank to fit down the road, then you might be better off getting one of the smaller false gars. Keep in mind that of all the true gars I have kept, they don't tend to grow steadily but in spurts. Gars can be trained to eat almost any food you would want to feed them, but when they are under a year, you want to basically free feed them (have access to food at all times) to prevent developmental problems. In my experience when gars reach a size of over 12" they will take almost any food you want to offer. You can even hand feed them. One other nice thing about starting out with a 20 gal tank is that once you upgrade, you can quarantine your feeders in it.