You can feed everyone an all flock all the time with oyster shell on the side and it will cover all ages well.
Any time you switch feeds (even you mixing feeds), there is a chance you will see a temporary blimp in production, I feel like I might see a drop in eggs for 2-3 days sometimes. Chickens are funny like that.
I wouldn't mix feed until 16 weeks old.
I would feed all, Hens and Pullets just Grower feed till Pullets are 16 weeks.
Then mix 50/50 till they start to lay. In the meantime offer Oyster Shells for the hens.
I feed a Non-medicated Start & Grow 18% or a All-Flock/Flock Raiser 20% with Oyster Shells separately after 10 weeks old, throughout Adulthood. Oyster Shells separately. GC