We have 4 hens that we adopted 1.5 years ago. Then we hatched eggs and have 2 Roosters- no hens- that are 5 months old. I'm thinking of adding 4-6 hens about 6 weeks old. They'll be separated for a couple of months until they are old enough to get along together. How long do I need to keep them separated disease-wise? Can they free range in the same areas? Will any diseases that they can pass amongst themselves be obvious before I merge the flocks?