Where did those pullets come from? That will have a lot to do with what your actual risk is. If they came form a closed flock, one that has not had any exposure to new birds for at least a month, they have been in quarantine, just not with you. Of course that begs the questions would the person keeping them recognize a problem if they saw it and would they tell you if they did. If they come from a flock that has recently been exposed to new birds the risk is much greater.
It is possible that your flock or the other flock has developed flock immunities. That's where they have parasites or a disease and are carriers but will not show signs of having them. Coccidiosis is a good example but there are others. No matter how long you quarantine them they are not likely to show any symptoms. One way to test this out during quarantine is to pick a potentially sacrificial bird from your flock and put it with the new birds. Then depending on who gets sick, if any do, you can see which flock is the carrier.
In my opinion, chickens from auctions, swaps, or shows are high risk and should be quarantined. Chickens from closed flocks are still a risk, but not as high a risk. If you can do a good quarantine it's still a good idea but very few of us are really capable of a meaningful quarantine.
