The issue with excess calcium from a layer formulation is cumulative (how much x how long), but most harmful to young chicks. As to why one of your chickens is laying and the other of the same age is not, might just be the breed. Some breeds will lay much earlier/later than others. And some...
Just going to add that starter and all flock are essentially the same for all intents and purposes. If the store doesn't have an all flock, starter will do just fine for all. Oyster shells still on the side.