The trigger and diodont (porcupine "puffer") will almost definitely NOT eat any soft corals. I have "heard" of a single instance of a balloon fish eating mushroom corals but that was it. I currently keep both a green spot puffer (widely regarded as being much nippier than diodonts) and a lagoon (Picasso) trigger with all sorts of soft corals, a candy cane coral, and several captive bred bubble tip anemones, plus more than a dozen small majano anemones, zoanthids, and mushroom corals. Hermit crabs, snails, and shrimp are also included in this tank.
You may want to invest in some macroalgaes such as Caulerpa or perhaps even vascular plants such as seagrasses or mangroves. These will be beautiful and will help reduce the amount of water changes needed.
Some of the giant hermits, especially Dardanus and Pagurus, preferably with symbiont Calliactis anemones, make great aquarium inhabitants. However, they aren't safe with corals (will eat leisurely if in need of a "snack"; have seen this happen in person) but generally are safe with fish and motile inverts. They are apparently quite territorial, so it is best to keep one per tank. If the individual is very large, it must also be fed with large pieces of raw seafood; this can be presented impaled on a stick, and the crab will pull it off.
As for snails, cowries, Astrea and larger whelks and conches can be kept. Cowries and whelks can be highly predatory on sessile animals, and some whelks also eat motile benthic invertebrates.
As for the diodont cruising the glass, as long as he is feeding it's probably not a problem.