Do not feed layer ration or provide extra calciumn until they are laying age. The extra calcium at a young age can hurt them, liver damage I believe. After about 6 weeks, switch to grower ration.
Purely my opinion and no specific knowledge or experience to back it up, but I'd suggest keeping them on medicated feed for about 2 weeks after they get outside, whether you have an existing flock or not. The medicated feed helps reduce their chance of becoming sick with cocci. It allows them to build up a resistance to cocci with less chance of becoming dangerously ill with it. There are 6 or 7 different strains of cocci. When they first go outside, they may come into contact with a strain they are not immune against.
As I said, purely my opinion. Good luck and enjoy!