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Don't think you can sex them. I think any two will be OK together, at least has been my experience. Your crab will be 100% safe with the butterflies. If he's in an anemone though that anemone might not be. Longnose butterflies though, I don't know, never had 'em. Supposedly they rank near copperbanded's in terms of "reef-safe-ness" but I'm not 100% certain on that.