Cowrie's are neat snails. I have a snake skin one in my reef and used to have a tiger cowrie in a FO
tank. The tigers get huge, (pretty sure this is what you were referring to) about the size of your fist and yep, you don't want one of those in a reef system. Fish only is great. They will clean up any left overs. I've read that they should be offered meaty foods but mine never ate anything I offered it. Never had a problem with it moving rocks around. I don't know what size tank you have but I wouldn't put one in a smaller tank just due to it's size and it needing enough room to move around unhindered. A cool thing about cowries is they have a mantle that they pull up to protect their shells. This helps keep the shell shiny and smooth. It's also an indication of it's mood. A happy calm cowrie will carry it's mantle up. If it gets stressed, unhappy from something irritating it or feels threatened, it lowers the mantle to expose the shell.