It could be a good idea to switch the new birds to medicated feed for a few weeks. It would be an relatively cheap and easy way to reduce chances of a problem with cocci. You could also figure out the dose to give corid/ampriprolium (liquid) in their water instead of the chick starter if you don't want to buy a whole bag of starter for a few birds.
I've heard of issues with people that quarantined new birds only later to have the new birds get sick and die suddenly when introduced to their own flock. Cocci can kill very fast if conditions are right and you add in stressed birds. Your own birds may even get sick could die if the new birds are carrying a new strain of cocci (some strains are worse than others). Though your birds will probably not have the stress factor especially if they are adult. Healthy adults will be less effected than younger chicks. Keeping coops/pens dry and clean is the best defense in the long run:)
Now all this might seem like we are being a little bit paranoid, but if you are unlucky enough to have a serious outbreak it can be heartbreaking. I've read so many sad stories here on the BYC about cocci outbreaks in chicks and adults where large numbers have died. I've only had a cocci issue once..thankfully I got it in check without loosing any chicks. These were "spoiled" brooder raised chicks that I put out on the ground during a wet/warm spring. I had switched them over to gamebird/all flock at the same time rather than keeping them on medicated starter . I ended up having to treat them with corid:(
Now I get a head start on building up immunity with my chicks while they are in the brooder by exposing them in small doses. I give them dirt and sand to play in and clumps of grass with dirt/roots while they are eating the medicated feed.