I have a Hammer's Cobalt Blue in my son's 20g tank. He's in there with 3 silver dollars, 2 tiger barbs, a gourami, a Black Mystery Snail and a Candy Striped Pleco. We've had him for about 3 months now.
I have NEVER seen Mr. Cleaner show any aggression to anyone in the tank, unless they try to take his wafer. Even when this happens, he only waves his claws in the air.
Mr. Cleaner dug a hole under the bogwood to hide in and the pleco, believe it or not, sleeps in there with him. Two days ago I plopped a 2" Mystery Snail in the tank. The crayfish's claws are small, but those shells aren't THAT hard.
He hasn't touched him and no fish is going to move slower than that snail.
I also would like to point out that these are Procambarus sp. crayfish and NOT Cherax sp. crayfish. Cherax sp. are very aggressive, nasty brutes and I will not have one in the house again. Tore up my plants! THREW the bogwood around! Dismantled my filter! Hated that guy.
Anyhow, this is not to say that all Procambarus sp. are the same, however they DO have a less violent nature as a whole.
Roan