Starter/Grower until 18 - 20 weeks, or they start laying. Then switch to Layer feed.
Scratch is a snack or treat. This should be no more than 5% of the diet. Not for chicks.
Oyster Shells for calcium, for the layers, not for chicks.
I keep a cheap little kitty feed dish (2 sides) in each coop. I put grit in one side and Oyster Shells in the other. They know if they need it, and will eat it as they need to. I keep them filled at all times. (In the Layer's coops)
Grit is not needed for chicks eating Starter feed, but when they are older and you want to give them treats they will need grit for that. Grit is small stones that the bird stores in its gizzard, where they act like teeth and are used to grind up food. Grit is necessary only if the chicks have access to grain or other foodstuffs. Chicks on mash or crumbles don't need it. You can get a chick-sized granite grit through your feed store or use parakeet grit from the pet store (for older chicks, not new babies).
Chicks really should not have treats, as the Starter feed alone is best. However, good treats for chicks would be boiled eggs mashed up finely. Nice for the extra protein, too. Yogurt is an excellent probiotic. Use plain, unflavored lowfat yogurt, about 1/2 tsp per chick. I mix it in with a little feed, and add a bit of warm water and make it like a mash. They love it. I feed it 2 or 3 times a week for chicks, and once a week to the older birds.
ACV is Apple Cider Vinegar. This needs to be non-distilled Organic; the kind with the, "mother," (looks like a glob) in it; not the white distilled vinegar from the grocery store. It is a light brownish color. Use this in the water, about a tablespoon per gallon of water to all ages. You can use it as often as you want for a host of health reasons. I usually use it every other day.
Hope this helps. Good luck with your flock!