I thought I would bump this thread with my recent experience. I have a Melanurus in both my home 120 mainly SPS reef and my office 90 LPS/softie tank. It should be noted that I really like these fish for the work they do, their beauty, and their personality.
The Mel in my 120 has grown to over 4 inchies over the last year+. He started nipping at the only clam I had in there (Deresa), but I got it in time and moved it (the clam) to another tank. Next it slowly made it's way through a lot of my snails, with the exception of the turbos. Yesterday I found that both my cleaners were gone. I've had them about 2 1/2 years, so I was hoping they were big enough for the wrasse not take interest, but I'm about 99% that is what happened.
So I guess I have three conclusions about these wrasses, other than they are beautiful and full of personality:
1) The bigger they get (and they grow fast) the more likely they are to take out your bigger shrimp
2) Clams are at risk
3) It occurred to me that many people who write 'he does not show any interest in my shrimp' can get a surprise on any given day.
Hope this information is helpful to someone.