I have had an H. chrysus that was over 5 inches in a mixed reef and he never touched a single snail or shrimp. I currently have H. biocellatus and H. melasmopomas in the same tank with no snail or shrimp predation at all. As
@cancun said corals are safe, just inverts on the menu. I think if you keep the smaller Halichoeres species your snails will be pretty safe. The big ones that get up to 7" are the ones that will be the most problem.
That said, some of mine have eaten feather dusters but the two I have now don't.