I've had my N. oerstedii (2") for a few weeks now, and she hasn't touched my large trochus snail (shell is about 1 1/4") and my medium sized scarlet hermit (the hermit's body is only about 1/2", but the shell is about 1"). The smaller blue legs and nassarius snails get taken out on a regular basis, as did a small hitchhiking crab of some sort. I'm thinking of adding a serpent star at some point to join the permanent crew, but keeping her well-fed with smaller prey seems to be enough to keep at least the trochus snail alive and doing his job.
So far she hasn't been interested in frozen food like silversides and shrimp, so she's getting spoiled with regular supplies of live stuff. I'm going to try to get her to eat mysis on a regular basis, which should also cut down on the cleaning crew casualties.