They are adorable.
I concur -- they don't need oystershell until they reach Point-of-Lay (~20-24 weeks more or less).
I don't give chicks an entire container of grit. I was taught to just sprinkle a little lightly on top of their feed -- as if salting it. They only need a very tiny amount.
I made the mash for all of them.
IIRC, it was a batch of a dozen so I used about a 1/4-1/3 cup of feed and maybe half a tsp of yogurt in the batch. The person who helped me said not to give too much.
I've only had to deal with pasty butt twice, both with shipped chicks.
I was advised to give them a wet mash of their chick feed, warm water, and just the tiniest dab of plain, unflavored, unsweetened, active-culture yogurt.
Also to put a little olive oil around the vent of the worst couple...