Bear in mind this is only coming from my direct experience, so feel free to take it with a grain of salt:
Almost all crabs are opportunistic omnivores. Most scavenge, some filter feed, some hunt directly....but with very few exceptions, few crabs of any size will fail to pass on a meal if it thinks its edible.
This holds true from the giant king crabs of the north pacific all the way down to the tiny little crabs that inhabit sponges and corals. It is also certainly true for the blue-leg and red-leg crabs we collect for our tanks. Blue leg crabs are definitely aggressive from time to time, but almost always their aggression stems from housing issues. I've seen them tussle with one another for shell rights, and I've seen them attack and pick to death a small unsuspecting snail....not for the food value, but for the shell. Well OK, I'm sure the food value too. But as for my own experience, I've never personally observed blue-legs bothering fish, seahorses, or pipefish.
I've had plenty of blue-leg crabs in my seahorse setups. Never once seen the slightest hint of aggression. I once had ONE red-leg crab - and not a big one - in my first seahorse setup. He was fine for months. Then when he was just a little bigger, I saw him casually reach out one day and try to snap a passing seahorse tail. Why would he do this? Food? Doubtful. Territory? Maybe. Because he's just a crab, and that's what they do? Probably.
I've had quite a number of blue-legs, as I say, and I've never had a problem. I think they're pretty safe.....but that's just one man's experience, and crabs have their own minds about what they will or will not try to eat. Add the crabs you think are safest, but always, always keep an eye on them.
I want to see them greeting and rising ^.^
