When i Brought My First Tank secondhand (75L rounded glass all in one) it came with a 4" RTBS who seemed somewhat happy in there, given a place to call his, originally a forked piece of wood pointing up he would swim around or just hang between the prongs and chase off anything that wasnt a catfish that came near then go back to what he was doing. When we modified the scenery for our son and made the tank into a bit of a dinosaur park (dino pic on back, ferny plants, plastic dino's in tank) he moved house to underneath a large plastic triceratops.
After I got my new tank I moved him accross (3ft wide) gave him a hollow log and his still territorial about that little space but he does seem a little happier (but that may just be being moved away from our kids banging at the smaller tank).
The main thing is you can keep them in a 20Gallon/75L or more with a space to eventually call there own (I replaced the large RTBS with a small one who shows no signs of trying to set up is own territory[likes to hang with the guppies], which from what I've read says they are mainly territorial more than just violent)
red tails grow 4-5 inches
rainbow or red fin sharks may grow up to 6 inches
hope this helps
Katchan