I find that having the potential food in the tank before introducing a potential predator can be important. It's almost as if the predator thinks the shrimp is part of the rockwork since "it was around when I got here" and ignores it.

Hard call on the dottyback/crab thing but I had an emerald one time that was catching all the other crabs in the tank and eating them. He ended up going back to the pet store.

The second cleaner shrimp is still around but has moved from a cave at the front to a hidden cave in the rock work at the back. It did not come out at feeding time. The dottyback did come out but did not seem too interested in eating!