I would guess that a clown would not host in a fake anemone, and I doubt that the arrow crab would catch one of them for a snack. I had a coral banded shrimp with 2 clowns in my 30g tank and there was never a problem, I think coral banded shrimps are more aggressive then arrow crabs. Sorry I don't have any real experience with your questions, that's just my 2 cents.
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