I have a maxima and engineer goby. The engineer buries corals sometimes but never buried the clam when I had it on the sand, although I did move it up a bit to a shelf so it got better light but thats the only reason i moved it
We had no trouble with sand sifters and clams, but the clams were med-large ones. Not sure if the smallest ones could get the sand off if they got dumped on. And IME, small clams are hard to keep anyway.
Haha, my engineer actually came from Cindy and Mike's tank but I bought my clam as a baby 1inch lil thing and it always managed to shed or move any sand away from its self if it didnt like it, but ya bigger clams would have to do that less I think.
I wouldn't worry about it much. I have one very stupid hermit crab that occasionally falls on my two clams. They close on it for a second and then shoot it right out. Kind of like a dumb hermit trampoline.
At this point I'm pretty close to moving the stupid hermit crab to the refugium. If I catch him doing it again, he's OUT!