Too big for sure. The Humu Humu or Picaso triggerfish can reach sizes up to 9 inches or more, I would suggest at least a 75, well really 100 gallon min IMO for the long term of a triggerfish such as this.
Whether or not your trigger would be too aggressive is dependent upon the individual fish's temperment primarily. Humu's are traditionally less aggressive than some of their counterparts such as a clown triggerfish or Rectangular trigger per say. They are notorious for becoming more aggressive with age though, much like grumpy old men do!
Remember individual personalities when dealing with a Picaso Triggerfish, observe the fish your thinking about getting before you buy, and ask the LFS what the fish's behaviour has been like if they have had it for some time.
On the otherhand... I started my first FOWLER tank "back in the day", which happened to be a 55 gallon too, with a picaso triggerfish, regal tang, porcupine pufferfish, cinnamon clown and a lunare wrasse and everything was smooth sailing for three years until I sold all the fish to a friend, but I did start with juvenile/very small fish and I may have just been very lucky with the temperment of my puffer and trigger, but I have seen many clown and picaso triggers put in very successfully with even a powder blue tang which is certainly no more hardy or defensive than a regal tang.
Good luck, but IMO only get the trigger if you want to upgrade your tank size within the next few years.