i buy squid, shrimp, octupus, scallops and put them in a blender.. make sure you wash the seafoods and cut the squid because the flesh is tough. lay out in a ziplock bag and freeze. i cut pieces out then feed them to the fish frozen
i have no experience with them except at restaurants. i just seen them on discovery channel and i would think they need a bigger tank and the care for them should be the same as octupus.
a 55 is too small for any grown trigger. the niger probably wont eat the damsels anyway. but if he does it is easier than catching them. i feed my triggers damsels once in a while. its better than feeding fw feeders.
it will probably get a foot long, keep in at least a 75,lots of lr and ls, feed meaty foods...i seen a queen trigger that was big enough to bite an arm off....looked like it weighed at least 50 pounds.....blue throats can also get large....