I once had a pistol with one of my watchmen gobies. The two with all there digging never bothered my clam, however that pistol use to snip branches off my xenia once in awhile
Like Deb and Rob mentioned, you shouldn't have any problems with the clam. I had more problems with the pistol snapping in the middle of the night-it use to freak me out as it kinda sounded like the tank was cracking
, other than that I'm sure you will get lots of pleasure out of watching them work together!