No need to panic. It will be ok.
Chick feed is more easily digested so they will be ok.
The oyster shell doesn't need to be put out until they start laying.
The grit and feed should NOT be mixed.
Separate containers for each of the 3 items.
I use metal rabbit feeders but any container can work so long as they can get their beaks in. In one coop I use plastic coffee cans with the side cut out. I screwed it to a wall stud so it cannot be tipped.
All of the above x2.
I use two methods for extra calcium: an old mixed nut container with a hole cut in the front filled with crushed egg shells then screwed to the wall and a 7# gravity feed container filled with oyster shell hanging from a chain next to the feed trough.
There is a second 7# gravity feeder on a chain at the other end of a second feed trough that has the grit in it.