either way, the purple may show some aggression with whatever animal you put in there since it already has it's territory set up and is a little more aggressive tang. definitely add anything at night after the lights are off. To truly avoid aggression, i learned a little trick.
depending on the size fish you add, go get one of those pool skimmer baskets (leave catchers) as big as you can find. attach it to the side of your tank, where the water flows through but not above it. keep the new additions in this basket for a week to get the existing fish used to having them in there.
and, last bit of advise, hopefully you QT anything you put in there, getting fish out of a 180 would be tough.
i'd bet you could get away with a lot of different fish because of the size of your tank. problem is the purple is not going to like them.