Hermit crabs, even reef safe ones, are almost all opportunistic omnivores. The advantage of the blue legs is they stay pretty small so they have a harder time attacking your snails, etc. Don't the scarlet crabs get somewhat larger?
Snails will die from starvation if you have too many of them. If they get knocked off the glass or whatever and they land on the substrate, usually they get set upon pretty quickly by the crabs because they are unprotected when on their sides.
Feather dusters, I believe, come from silty/muddy substrates (I assume you're talking about the big ones). They make their tubes out of that real fine silt. In a reef tank, there isn't really a similar substrate. I put mine in the sand and they seem to be OK there. The big problem with feather dusters is lack of food. Unless you have an established tank with lots of plankton, or you're feeding plankton of some kind, they seem to slowly starve. I've read that they should get enough food just from feeding the tank, but that doesn't match up with lots of stories I've heard and my own early experiences.
-Jim