For what it's worth, I just bought a 2" male melanarus wrasse from my LFS. I had asked for their help in finding a solution for the high number of spaghetti worms in my tank. I have four shrimp: A skunk cleaner, two large peppermint, and a fire shrimp.
The LFS' steered me away from looking at a Copper-banded Butterfly in my SPS/LPS mixed reef. They told me the CBB would most likely end up picking at my duncans and other LPS with soft parts. They also cautioned me about CBB and SPS with extended polyps.
They then recommended the Melanarus as a possible eradicator of the spaghetti worms and went through a series of questions in regard to my concern about keeping a Melanarus with shrimp:
1. Does your reef have a healthy population of pods? Answer: Yes, I have hundreds in my refugium and main tank. Reasoning: Melanarus wrasses will hunt pods before going after large shrimp.
2. What is the size of your shrimp? Answer: They all are at least 2" - 3" in length without antennae. Reasoning: Melanarus wrasses won't bother shrimp that they are introduced to that are the same size or bigger.
3. What is the nutrient load in your tank and how do you feed your fish? Answer: I carbon dose so that I can keep my fish fat and well fed without having nitrate/phosphate issues. Reasoning: A well fed Melanarus will not bother shrimp, especially big ones.
I'll report back what my experience is. At this point, my Melanarus is looking over the rocks and pecking at things (I'm guessing pods) in between feedings. I love my shrimp, but with my huge rock stack, they are rarely front and center. In fact, I've named the Fire shrimp Howard Hughes. He's locked himself in a cave and I only see glances of his white antennae. On the hand, the Melanarus is front & center and BEAUTIFUL! In the grand scheme of things, I would prefer to have all of them. However, if I had to pick one or the other, I would take the wrasse at this point.
Only you can decide what your risk tolerence is, but I don't think it's as simple as "you can't keep a Melanarus and shrimp together." There are several factors that impact the risk to shrimp.
Hope that helps.