Where are these new pullets coming from: a hatchery, feed store, or somebody's flock? How old are they? If they are newly hatched from a hatchery or feed store I would not quarantine. They are highly unlikely to bring in anything. If they are older or from another source then I'd consider quarantine.
Different diseases or parasites can spread by different means. Some are spread by sharing a run or eating or drinking out of the same sources. Some are spread by certain vectors like earthworms, grubs, grasshoppers, or such. Mosquitoes can spread fowl pox. Some, like Marek's, can blow in on the wind. You can spread some, maybe on your clothes (especially shoes) or by using the same buckets to carry food or water to them. Even storing their food in the same containers might carry a risk. The more you can isolate them the more things you can protect them against but most of us do not have the facilities or ability to do a perfect quarantine.
A flock can develop flock immunities. No matter how long you quarantine them they may not show any symptoms. It could easily be your flock that has developed flock immunities, they may infect any new chickens.
How isolated is the flock you are getting them from? If they have had no contact with outside chickens for a few months they have in effect been in quarantine. The questions are how well do you trust the person selling them to tell you if they saw a problem or would they recognize a problem if they saw one. If they have been exposed to outside chickens then all bets are off.
When people quarantine older chicks (older than from a hatchery or feed store) they often treat for mites and lice and maybe worms. It will not hurt them and could save you some issues.
1. Seperate coop...quarantine how long? 2 weeks?
As isolated as you can. I agree, 4 weeks is pretty standard.
2. Do I keep them seperated from older hens, but behind wire so they see each other? Or completely seperated until after quarantine?
Completely separated until after quarantine.
I don't. That method can work, especially if they have a lot of room in the coop and if you are out there at daybreak when they wake up to let them out. They are going to determine the pecking order. Sometimes that is fairly peaceful, sometimes not. If they are left locked in a tight space any violence can be magnified.
I like adding them during the day where they have a lot of room. I don't know what your run looks like, if that qualifies as a lot of room. Maturity levels can make a difference also, that's another reason I asked how old they are. With plenty of room it can often work well to just turn them lose. If room is at all tight, housing them across wire for a week or so can help a lot.
You have two different situations. One is during the day. Especially if the new ones are immature you may have two separate flocks for a while, the younger avoiding the older but the older not attacking them. At night, my juveniles do not sleep on the same roosts as the older but find somewhere else to sleep. The size and layout of your coop can be important.
Good luck! People go through this all of the time, usually with success. But it is possible to have a disaster.
