I have had lots of success with surrogate silkies. If she hasn't killed the chick as of yet, I don't think she will. I wouldn't separate mama and chick. Is she keeping the chick warm under her feathers and sitting with her? If she is I think she will continue to care for the chick. Good luck! :)
Depends on the bird and the breed...
Some start earlier around 5 months and some can be later like my buff silkie who was 1 year old when she laid her first egg :)
I would go with the Pyrenees. They are guardian dogs. Herding dogs are not a good choice from what I have read. No matter what type of dog you choose, you should introduce slowly and keep an eye on them for the first while as some dogs may be very tempted by your chickies. I have a mutt and he...