Triggers get too big for a 30g and very mean also.....won't take long for the fish to start killing any other fish in the same small tank, and don't forget that crabs, snails and shrimp will be on the menu too.
Blue damsels can be quite mean as well. To be honest, I wouldn't cycle the tank with anything alive, just toss in a piece of raw shrimp in the tank and let it do it's thing. Not only does it save the life of the fish, but it will save you the frustration of catching the damsel when the cycle is finished. Because catch it is what you will have to do if you put the blue damsel in first! They are very territorial and will claim the whole tank. Woe to any fish that you put in there with damsel already in residence. If you really want to have a blue devil damsel (and don't get me wrong, cause I love damsels) you will need to figure out what fish you can have in there with it, one that will be able to deal with the damsel and more than likely put the other fish in the tank first, give it time to get accustomed to the tank, like a week or so, then add the damsel. The only exceptions to this advice would be fish that can hold their own against the damsel, like some of the dottybacks, even bigger, meaner damsels, or a dwarf lionfish (which damsels won't mess with!). What kind of fish did you have in mind? Is this going to be strictly a fish only tank, or do you want to have inverts and corals?
I'll second the live sand suggestion......oh and if you are going to keep ANY kind of inverts, it's best to keep your tank at a salinity of 1.025 or very close to it.
