if the peppermint shrimp is bigger than the crabs it
should be ok. as for using the juice, i'd tend to think a natural solution to the problem, like peppermint shrimp, would always be the better choice... provided of course it doesn't end up someone's dinner
anyway, my one peppermint ate probably 15+ aiptasia in a span of a few weeks. now that they're gone, i just hand feed him some krill now and again -but he's always out at night scavenging. so i wouldn't worry that it won't get enough to eat.
just my 2 cents...