My dad had one in his 180g for years. He was my favorite fish ever. He was an eating machine! He would eat anything and everything 24/7. He kept him until it was about 9" long and then gave it to the local zoo who had it several more years in their giant tank. I can't fathom being able to "train" them. I just think you are really lucky if you get one that doesn't touch your coral (or your arm when it gets bigger). I'll keep my fingers crossed for you, it is hard to make a decision--fish vs. corals. I am having that problem with my sailfin tang right now who has all of the sudden decided that corals are fun to play with, carries them around, picks them, tosses them. aaaggghhh..
We brought a spiney boxfish puffer home from the ocean last year because I couldn't resist him. He was sooo cute, about a 1/2" long. Big mistake! It was the beginning of an ick nightmare! At least yours is in the 54gal for now. Obviously, not good to get ick anywhere, but better than your big tank.