The worst is if you lose one and then have a flock of one. I started with 4 but through 3 years of damn cats, raccoons, and heat wave we only have one of the original 4. I would try to get birds close to the age of yours or slightly younger. It is much more work to integrate adults than it is when they're younger and more adaptable. If you can't find birds locally some of the hatcheries will sell older juvenile pullets singly. You could order 2 or 3 to keep your flock a manageable size. Make sure you isolate so the newbies don't bring in disease. Best of luck!