Percnon crabs?
 

I was at a LFS and in one of the tanks were some crabs with the label of Percnon crabs. After a little googling I found that this was the scientific name for Sally light foots. I was thinking of picking one or two up. I just wanted some feedback from anyone who has kept them. I have about 50 nassarius snails in my tank and I wouldn’t want to see them disappear one by one. How aggressive are these crabs? And has anyone had any trouble keeping them in a reef?

Thanks for any input