Lots of opinions on the food. This is just what I do. I have had perfectly healthy chickens, great layers, and zero mortality, FWIW.
They get the chick starter for 2 months or 9 weeks approximately. I begin to transition them over to a grower type, nice high protein, from 9 to 16 weeks. I don't start the layer pellets until they are approaching POL, ie, point of lay. It's rich in calcium and I don't wish to over do it too early. Option 2 is to simply leave them chick starter straight through until 18 weeks.
That's it. That's the program for hens. Meat birds never need the layer formula.
Scratch is french fries and McDonalds for chickens. Yup. It's "junk food", (as I duck for incoming flaming). But seriously, it is nutritionally inadequate and if your young birds fill up on it, they eat too much, they poop too much, and they potentially can pick on each other too much due to protein deficiency.
Scratch has it's place, in very amounts, as a behavior modifier, but NOT as food, if you want egg production, it simply is inadequate to make up more than 5-10% of the hens diet. You'd be far better off with green grass, bugs and other natural foods that birds find in their browsing about. Of course, these are tough in the winter months, sadly. Best regards.