Chick starter should have grit in it already. Oyster shell and added grit should be given free choice in separate bowl. Your chicks when they are 4 to 5 weeks old and out side will likely dig their own grit but can be given free choice grit also.
I e-mailed Purina a while back and asked this question. Chicken feed, whether Starter, Grower, or Layer, does
not have grit in it. The feed has already been ground up. The chicken's gizzard can break it up without the benefit of grit.
Joe.G. you do not have to provide scratch. They will do fine without it. If you decide to provide scratch, feed it in small amounts. Since it is an incomplete feed, they will do better eating mostly the well-balanced diet provided by the chick and chicken feed. Too much scratch can upset that balance of nutrients they need for best growth. They can survive and grow with a lot of scratch, they just won't have a balanced diet and possibly won't be as healthy. When Dad raised chicks in a cardboard box brooder many decades ago, all he fed them was corn meal until they were 3 or 4 weeks old and could go find their own food. They lived and grew, but they would have grown better and developed better with a balanced diet.
If your birds free range, they should be able to find their own grit, but it will not hurt to provide it on the side. Just expect it to last a long time.
I strongly agree with Tivona and Clay Mudd on not mixing the grit, oyster shell, and feed. Let them self-regulate.
There are plenty of studies that show too much calcium can harm growing chicks. In those studies, they cut the chicks open to see what damage is done to their internal organs. But they are not going to drop dead the instant they eat a bite of oyster shell or Layer. What harm might be caused depends on how much total calcium they eat over a time period. When I have young chickens with a flock, I feed them all the same food (Starter, Grower, Flock Raiser, a combined Starter/Grower, a Grower/Developer, depending on what is available and the age of the chicks) and offer oyster shell on the side. The ones that need the calcium eat the oyster shell and the ones that don't may eat a bite or two but they generally leave it alone. When mine free range, they don't eat much oyster shell anyway. They find a lot of calcium in some plants and bugs they eat and can even get calcium from the gravel they use as grit if you are in limestone country.
When chickens eat about anything other than prepared chicken feed, they should have grit. They don't have teeth and use grit in their gizzard to grind up hard or tough things like grain and grass. When I have chicks in the brooder, I take some sandy dirt and put it in with them, offered on the side, so they are prepared. But if you only feed prepared chicken feed, you don't have to give them any grit.