My teenage son wanted a flame scallop even though the LFS told him it wouldn't live very long. In fact, since I had purchased my very expensive system there a couple of months previously, they gave it to him for free. That was about a year ago. (I think they were charging about $15 for them here in northern California.)
The scallop will indeed move regardless of where you want them. Ours has moved itself into a corner where debris can collect. We have a lovely large sump stocked full of articpods and will also use phytoplankton. When I hand feed three times a week I usually squirt some mysis shrimp, small meaty bits, or cyclopeeze in its direction.
It started out just slightly larger than a quarter and now it is bigger than a silver dollar. If Reefkprz is correct, it will be dead within 2 years. But it has a reasonably good life other than when a clown tries to host it.