I had one in a 40 breeder. I think it would be fine in a 55 provided you had adequate filtration for such a pretty monster and fish that could live without it eating them.
If you ever kept it long enough to outgrow your tank, I'd be proud of you. It would take a while, and by then you'd want a bigger tank anyway.
I had mine with a lion fish and a tang and a couple other things. I didn't know as much back then (92!) and wouldn't put as much in a 40 now, but everybody was healthy. You can see pics in the link off the homepage.
I like those triggers too, Picassos aren't as
crazy as a Niger Trigger, that's for sure.