I only answered the first one.... in the beginning....
One thing that dictates my "supplementation" is my climate. I live in the High desert. There is no grass to speak of. I cannot allow them out to free range because of coyotes. At the time I had about thirty to thirty five chickens...
first and foremost before you feed your chicks treats they need to be on grit. A broody hen would offer up tidbits and they would be eating a bit of dirt from the get go. But if they are raised in a brooder....
Some people put in a clump of dirt with grass attached for them to explore...