in most cases, red legs are bigger than blue legs, so the blue legs usually know not to mess with them. I've never seen emeralds fight anything, but I definately think they could hold their own.
another note on crabs, if anybody evers gets a chance to own a porcelain crab, DO IT, because they are awesome. I got one off this site over a month ago. I generally don't have a lot of luck with inverts. after a day, the porcelain went missing and I later assumed it was dead. earlier this week, it shows up on a rock, eating away. it stayed in the same spot for 2 days, eating and watching the fish swim by. it's cool to watch it eat, and in my opinion, it is by far the prettiest of crabs. if/when I order from here again, I'm going to get 5 more.