I give them oyster shell free-choice. Started putting it out in a hanging cup next to the grit cup at around 18-20 weeks, but I could have put it out earlier, because they only started to eat it when they needed it, and each chicken was different in the timing. How they know when is just amazing. One day you'll just see them scarfing it down, when before they tasted it and went "meh".
I feed them crumbles, they like pecking the little bits and it takes longer to eat, plus I make a mash of it a lot of times. It's messy in the bowls and cups, though they peck the bits that fly out, but works great in the deeper trough feeder. I'm not sure they wouldn't beak out pellets anyway. Queenie and Butters both get a good hoe-ing, sweeping-toward-them motion going, like they are sorting it out for the best bits. They do that with everything.
Buckeye chickens are a heritage breed and dual-purpose (eggs and meat), so I didn't get the usual feed for them growing up. I followed feed guidelines from Shumaker Farms (a Buckeye breeder) and the Livestock Conservancy
www.livestockconservancy.org. The Conservancy has a chart called "Heritage Chicken Feed Guidelines". Both they and Shumaker Farms call for higher protein from chick age to point of lay and even beyond, compared to modern hybrid chickens. The Conservancy says it helps them with "heat/cold stress, and better maintains condition of the heavier bodied heritage laying hens." So they got Gamebird Starter/Grower and are now on a high-ish protein All-Flock crumble. Extra calories for the colder months without getting too much protein - I offer slightly more treats.
PS
@BY Bob - Today 3 separate little bowls for cooked oatmeal worked well in staving off piranha-like behavior...