I was accidentally shipped two flame scallops yesterday and after doing some reading (luckily, I had looked into them before) I opted to give them to my LFS. They are filter feeders and require a very established tank for survival. Supposedly, even in an established tank, their life expectancy is around a year. I'm sure people have anecdotal evidence of happy, healthy scallops, I'm just telling you what a majority of the literature reports.
Scallops "swim" through propulsion of shooting water out the front so they seem to swim backwards (learned this on Discovery!) They will oftent end up in a spot of the tank they can't get out of and out of view. the best thing to do is to place then at the front of your tank in an area with good flow. They do secrete material to "connect" them to a rocky surface, so if they're happy, they'll stay. I'd be very interested to kinow how yours do. They are cool additions to the tank. Hopefully this helps.
PS - SWF.com told me I'll need to add an invert additive about once a week to support the scallop as well.