Okay, I need to know if it would be practical to put a red tailed shark by itself, as the only fish in a tank, knowing that they can get big and territorial as they get older. Any advice?

depends, how big is your tank?

I'm still working on that, unfortunately. I'm thinking 15-20 gallon, but I wasn't sure if having the shark as the only fish would allow for a smaller tank.

Get a 20 gallon so it will have enough room to grow and thrive, other than that its ok.

Hope this helps-petsmart-redtail shark

a 20 should be good for him alone, as you have already said, they do get territorial. also, if you add anything else he will terrorize them until they die, its not really worth having them in a small tank with tankmates.