I'd stay away from the red coris with a 90. They have been known as tank breakers and can get up 12" in length. I think it would be hit or miss (with many more hits) on whether it goes after the clams and IMO I wouldn't take the risk with a full blown reef tank. They also mess with live rock and flip things over.
That being said, I have almost a full grown coris venusta and I'm testing him out slowly with his first clam. He doesn't mess with anything, however, which seems unusual. At the LFS, he was housed with crabs, shrimp, and small fish (they don't know much, they didn't even knew he hid in the sand). I've had him two months and no snails or sea serpents (2) have gone missing or been injured. Granted, this species is not as pretty, mean, or large as the coris gaimrad:
<img src="http://i12.photobucket.com/albums/a240/techigirl78/coris_venusta_1_18_06.jpg" alt="Image hosting by Photobucket">
I'm hoping the best for him. On day three, it still doesn't seem to care about my new clam.
As mentioned before, I'd probably look into another genus that is likely to be beneficial to your clams versus a threat.