I wouldn't be mixing the oyster shell into their food. I'm not sure if there are consequences from ingesting too much of a calcium source, but that would be my concern. They're not going to be able to tell the difference between that and their food and I imagine they'll inadvertently be eating more of it than they otherwise would. A free choice dish on the side is plenty. I'm glad to read you've got feed now and the niacin supplement going on