all triggerfish need bigger tanks than that. I've seen it recommended 55gallon for a Humu, and that's about the smallest tank required for any trigger. most require 75-150 gallons.
with your tank, you could probably add something like a royal gramma, a cardinalfish, a damselfish, possibly a small dwarf angel, a dragonet (depending on how well your live rock is established). those are some ideas. I definately think that tank is too small for any trigger. most triggers grow to 9 inches or more and can be quite violent with peaceful neighbors.