Damsels can be very aggressive, but it all depends on the fishes personality. Has he attacked anyone already? If he has that's a big warning sign. I have a blue damsel myself, and even though he was in the tank first, he's never been aggressive towards any of his tankmates. As far as the humu trigger, a 75 should be fine and you can have them with a puffer or tang as long as the humu isn't bigger then them. If he is, then when you drop them in, he'll be thinking lunch.
Here's a little info on the Humu:
Saltwater Aquarium Fish for Marine Aquariums: Humu Picasso Triggerfish
If you get a trigger or a puffer, then you won't be able to keep shrimp/snails/crabs as they will become food.