I have a Huma now and in the 20 years I have had a tank, I have had 3 or 4 others and call me lucky or what but NONE of my Picassos have been the total aggressor that alot of people make them out to be. I never noticed Huma sized bits missing from his tankmates. First and foremost, Keep them fed. Hey I get irritable when I am starving also, and sometimes a nice piece of fish looks good to me! Remember in the wild whenever they get the slightest bit hungry, they eat. Starve a Trigger long enough and his eyes get bigger to the size of your tankmates. When they are fat and happy, they cruise around, do exercise laps, hunt and in general just have a good time. BUT if they are starving they even will hit on dead coral to try and get a morsel, then they will turn to you to say "I meant to do that!". They are smart, give good eye contact and are fun to watch them make new sleeping spots. When I walk into the room, he is the first to swim over and acknowledge me! Way cool.
Second, get a small one that has not been starved or abused yet. Alot of the bigger ones you might come across have already gone aggressive due to bad environment or care and they are there because somebody got rid of him for being a butt-head. Get a nice mellow small one, de-louse him and keep him happy and fed and you will have an excellent tankmate.
When I use my cleaning magnets, he chases it and even does the grunting sound Humas are known for. I have a lot of fun and enjoyment from my Picasso. As much as I like the looks of the Clown Trigger, I would rather have a Huma any day.
They are really hardy also, if something goes really wrong with the tank, he would be the last one to go. I had one jump out of a tank, flop on the floor, pick him up with my hand and he lived for years after that.
After all that is said and done, fish are like people also. There is an exception to every rule. A butt-head in every crowd. I just hope you are lucky like I have been.
Good luck,
Glenn