Might be a bad idea. Adding triggers of different sizes and aggression levels can be bad news for the small guy, double in your case. I have a clown and a huma huma with a dogface puffer. Feeding time can get a bit rough, but otherwise all is well, I think because the triggers are equally aggressive.
But the puffer rules the tank.
And I would love to see a 20 inch trigger, the biggest one I've ever seen was about 10, at the Shedd aquarium.