I have been feeding the Manna Pro Oyster Shell that I get from TSC, which sounds like what you used to feed. Where did you get the gray shell that you are feeding now. I didn't realize that the type of oyster shell mattered.
I don't know how many chicks there will be, but when I got my 12, I followed advice from Blooie on this site. What about making the mama heating pad?https://www.backyardchickens.com/articles/brooding-with-mama-heating-pad.67728/