I'm not sure on the exact percentages, but layer feed has (I think) double the calcium of grower. Non laying birds do still need some calcium, just not as much as an active layer.
I've had a mixed age and gender flock for years and years. I feed an all-in-one, similar to grower or Flock Raiser. I offer egg shells and oyster shells in the run. Yep, littles have access to the eggshells and oyster shells. Thing is, they just don't eat it. I mean, they usually try a piece or two, but they find it's just not to their liking and don't eat any quantity of it. Same for the roosters, they'll get all excited and call the hens for it, cause they know the hens like it, but don't eat any themselves.
It's kind of funny--I toss scratch in the run, everyone comes in a huge free-for-all. I toss oyster shell in the run, they all come running until they know what it is, then half the birds walk away disgusted while the hens peck happily away