Humu's/Picasso's should be in a tank of at least 60 gallons, if not more. They are very active fish, and need a lot of swimming room, even when small.
They are great fish with an unbelievable personality. I am completely enamored by mine. Mine never messed with my shrimp if I put them in after lights out, or if they were in the tank before him. Otherwise, forget it. He leaves the crabs alone, but the snails are fair game. Need to replenish those every so often.
I've tested him out with some corals (leathers), and he nipped at one toadstool, and then left it alone. My Koran on the other hand almost killed a colt coral that was in there.
One other concern with having a trigger in such a small tank is water quality - since they really should be in a FO tank, you will need a good strong skimmer and do frequent water changes to keep the nitrates down. If you are going to keep him in a small tank for temporary purposes, make sure you test your water params frequently.