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Don't qoute me word for word here but I am pretty sure blue legs are reef safe and scarlets aren't. Blue legged hermits tend to find a lot but are reef safe.
We posted at the same time. With opposite advice. I hope someone else will come in as the tie breaker. The scarlet crabs are normally called scarlet reef hermits.
both are reef safe. blues stay fairly small. true scarlet's can get to be about an inch, so the only concern there is making sure you have spare shells large enough for them, so they don't evict a snail (typical if the snail gets over-turned)