I've read they're aggressive toward other blennies, dragonets, pipefishes and seahorses. Other than that, he should be fine. Most other fish ignore them. My blue damsel hasn't looked at my newish blenny twice since i introduced him. As far as tank size, they can get fairly big (six inches or so). It depends on what other fish you have, I guess. They also need plenty of algae to eat, so plan on him constantly grazing your tank. That should give you a good indication of how much room you'll need. If you're planning that 10.5 gallon, you'll probably have to give him away when he reaches full size. Most are sold (from what I've seen) at around 2-3 inches. I would think one would be fine in your tank for the time being.