Porcelain crab might be pushing it, especially since they tend to be smaller than the decorators. I had a coral banded shrimp and the decorator ate him, but the shrimp may have been on his way out anyways since the day before the decorator got him he was acting erratically. But yeah, the decorator is like every other crab in that they're aggressive scavengers. I think it helps with mine that I feed him a piece of silver side every day while feeding my clown fish that he's in with, whom he's never bothered aside from raising his claws at them to shoo them away when they got too close to him.
P.S. I wouldn't go with the anemone just yet, they require better conditions than a 6 week old tank can offer. But the zoas should be fine, but I'd go with a $10-20 frag, since you never know how they'll do.