Many feed mills do a starter/grower combo. Especially here it hard to find them separate. You can do starter 4 -6 weeks and do a switch over gradually over a week or two by mixing it until you have them on just grower. Keep them on the grower until you actually see an egg from the pullets then you can switch them over to layer feed and start offering extra oystershell.
By the time your chicks are big enough to go in with the grown hens they will be near point of lay. Chicks need to be at least 16 weeks old and close to the same size as the the adult birds before they are big enough to defend themselves against the attacks that will come being the 'new chickens' in the flock and the establishing of the pecking order. At that time you can put out the starter/grower with the layer, they will all eat it.
Hens can be very mean. Yhey can prevent smaller and/or younger chickens from eating so that they starve and they can intimidate them and leave them hiding. They can also pick on them until they have injuries that are fatal. The younger chickens are better served being as close to the same size as the older chickens simply because the playing ground is more even and they can defend themselves and fight back if needed.