Welcome! It's fine to feed an all-flock feed forever, with oyster shell on the side. It will have a higher protein and general nutrient level than layer feed, a good thing IMO.
I feed Purina Flock Raiser to everyone, all ages, layers and not, and it's been a very good choice for me.
Good food pays off!
Select something that has a recent mill date (on the bag somewhere, within one month) at your local feed stores, and get a quantity that you will use up within a couple of weeks. After about six weeks from the mill date, it's getting old!
If you plan to feed layer, wait until all the pullets are laying eggs, and have oyster shell on the side.
Chicken feed is cheap/ pound, even the good stuff!
Mary