Shrimp are not crayfish. It is true they are both crustaceans, but a crayfish is a crayfish and a shrimp is a shrimp. Some shrimp are misleabeled and sold as ghost shrimp and they are predatory, but not very able to take down a whole fish much larger than themselves. This isn;t very common in my experience, but I have heard of it. Amano shrimp are not likely to be confused with these shrimp, so I think you're pretty safe. Just chalk it up as more crappy LFS advice.
If you're still worried, I keep ghost shrimp and love them. I have two tiny feeder guppies i inherited from a friend who was moving out of state and they are tiny fish. The shrimp I have are actually bigger then these guppies yet the guppies have not been attacked and eaten. So, there you go.
