Feeding almost layers and older babies

I have 9 "almost" layers (expecting some eggs within the next week or two) and 7 "babies" (layers too) that are 4 weeks behind the older layers. I need to buy more feed (which lasts about 3 weeks with this flock). Currently I buy Purina Flock Raiser (20% protein) but I wanted to switch to a 15-16% protein Layer Ration when they started laying. Should I just go ahead and buy the Flock Raiser until they are all laying and just offer oyster shells? Will I have issues with the "babies" eating the shells? Any other recommendations? Maybe I'm making a big deal over a 4 week age difference but this is my first flock.