I never had a problem with my CBS and my hermits when I still had the hermits. But after I got the peppermints I relegated him to my sump because I was told that they would get territorial and fight.
ive only heard they might be a bit dodgy with other shrimps.....never heard of any issues with crabs.......on that note i had 2 pepermint shrimps and when i added 3 cleaner shrimps the pepermints disapeared within a week.