Welcome to BYC and congrats on your new chicks!
Grit can be offered from the start - while commercial feed is water soluble and does not require grit for digestion, there is an advantage to using age appropriate grit from the start to help your birds develop the large, strong gizzards needed later for digesting food other than that water soluble feed. Definitely introduce grit before feeding any non-soluble feeds/treats.
Oyster shell is only needed when the birds reach laying age -- even if you choose to feed a layer ration that is enriched with calcium having the oyster shell available is an added "fail safe" to assure your flock has the ability to take in the calcium they feel they need from day to day. I don't use layer ration, so I use oyster shell as the calcium source for the birds in my flock who are actively laying. The nice thing about oyster shell is that only those birds who *need* it are interested in consuming it -- other birds (chicks, roosters, etc) may pick at it at first out of curiosity, but they don't consume it like the actively laying birds don.
