My broody is due to hatch 5 eggs on the 7th and I was thinking of getting medicated chick starter for her chicks, but I was wondering if mama ate it would it affect her badly, or would it immunise her or what??
Assuming the medication is amprolium, it won't affect her. It stays in a chicken's digestive tract and acts on the cocci they eat when they eat from the soil. It doesn't even affect the eggs. However, normally a broody's chicks pick up some immunity to cocci from the mama (from eating some of her poop.)
So, if you prefer to feed medicated, that's fine. If you don't, that's fine, too. What you DO need to be sure of is that the chicks don't eat layer feed -- which they will if they can, and mama will feed it to them, too. The extra calcium is quite harmful to their organs. The mama, on the other hand, can eat starter, grower or flock raiser. She won't even need the extra calcium in layer for a while, because she won't lay while raising her chicks.
My flock varies in age from a couple of weeks or so up to 4 years. I feed them all grower or flock raiser. I offer oyster shell and grit separately. The chicks will dig out small pieces of grit but they leave the oyster shell alone. Works fine.
I will feed them the medicated feed just in case, I'm so worried that something might happen Like them getting a horrible disease such as cocci!!
I will let mama raise them with the flock, but will keep a close eye for the first few days, both for aggressive hens trying to get her chicks and for chicks wandering off etc. I think I will feed them all starter, grower etc and put oyster shell aside like you so with you flock.