None of my guys pick on anything. Not coral, not the clam... never see any nips on anything.
I will say this.
I have learned to only buy LARGE clams for a few reasons.
The smaller ones are dependent on both micro foods AND light. They are more choosey as to the micro foods that they will accept and thrive off of. The larger clams are pretty much photosynthetic and I am told that they are less choosey as to the food that they want/need to live healthy.
This has been my experience, as I have purchased a few of the smaller clams over the years (around 2inches) and they did not live well for me, no matter what I fed. I had MH lighting for those clams too.
Secondly, I have been told that the larger clams can withstand "normal" nips to the mantle. They can regenerate very quickly and will be totally fine. The smaller clams are easily upset and once they are nipped at, they can sometimes become "shy" for long periods and will not heal quickly because they won't open up to accept food or enough light.
This big clam has been really hardy for me. He moves well and opens up really nicely, fish swimming over or my hand's shadow does not phase him and make him "shut" as much as a smaller clam.
My mother bought the "brother" to my clam. Same size, same type, from the same shop. She kept a regal angel and a coral beauty with hers. Never had an issues. She has a rock beauty with it now and still no issues.
I would say that if you were going to do it, get a cheaper clam, like a squamy... and get a large one. If it gets nipped, it should heal as long as it is healthy. I don't think it would be worth it to get a smaller one. Try to find a 4inch + guy. It will have a better change of surviving. I think it's better to spend more, for this reason. This is what I would do, but that is just me...
Also, for what it is worth, I don't feed ANYTHING with clam in it. I don't want them getting a taste for it. I make my own food and put nothing clam in anything. Just a thought for you...
Now I am gonna upload a LOT of pictures.