How old will the pullets be when you plan to add them? Are they going to be chicks or pretty much grown? That makes a big difference.
There are two different reasons you might separate chickens for a while. One is quarantine. If you are bringing in new chickens from a source you don’t fully trust (that includes practically any chickens other than just hatched chicks from a reputable hatchery), it’s possible they are carrying a disease or parasites. Chickens can transmit diseases through sharing drinking water, through their poop, or even just through the air. A proper quarantine involves housing them far enough apart so they cannot even share air, you use different buckets to carry feed to each group, and you change shoes when you go from one to another so you don’t carry a disease on your shoes. The idea is that if you separate them for a month and they are sick, you can maybe catch that before you mix them with your flock.
One flaw with that method is that many flocks harbor a disease yet are immune themselves. Coccidiosis is a great example but there are others. No matter how long you isolate them, they will never show any signs. It could even be your current flock that is the carrier and has the flock immunity. Don’t get me wrong. Quarantines is a great tool, both to show parasites and allow you to treat them and to expose anything those chickens may have recently been exposed to and have not built up an immunity to. I highly recommend it for chickens you get at a swap or where they have recently been exposed to other chickens. Even the stress of the move may cause them to exhibit symptoms you normally would not see. But quarantine is not a perfect tool.
To me, a better way to do it is to pick a potentially sacrificial member of one flock to put with the other flock, the flock that has the least value to you. That way, any diseases one flock has developed immunity to is much more likely to be exposed.
Many people ignore quarantine and generally don’t have big problems, maybe they have to occasionally treat for mites or lice. But occasionally a flock is wiped out by a disease. A lot of that decision is whether or not your facilities allow you to quarantine, how badly you want the new chickens, and how precious your current flock is to you. With baby chicks from a hatchery, quarantine is not even on my radar. I just don’t worry about it.
The other reason you might want to separate chickens for a while is totally different. Chickens recognize which chickens are members of their flock. Occasionally a chicken may attack a strange chicken that does not belong. This doesn’t happen all the time but it happens often enough to be a concern. If you house the chickens where they can see each other for a week or so, it can make integration go much easier since they at least recognize the other’s right to be there. This doesn’t always work, they still have to sort out the pecking order, but it can really help make integration go smoother.
If you get baby chicks, you’ll need to brood them until they can handle the weather on their own. The more age and maturity difference there is between your current adults and the new ones, the more risk there is when you integrate. That’s why knowing the age of the ones you plan to add is so important. It makes a huge difference in how to go about it.
