I would recommend atleast month or two, as you might not see a health issue in just two weeks and thus infect your whole flock. Sadly this has happened far too often and whole flocks have died or had to be culled.
Of course they are your chickens, just think on this for a bit...why are the chickens unwanted? If you have a healthy flock, why would you want to take the risk of bringing in a disease, that might not even show up in just two months, to your healthy flock?
Bringing in chickens, whilly nilly is really really risky and most find that later, they regret their decision and have some really hard choices to make regarding their whole flock, not just the new ones. Once a flock is exposed to disease, it is too late but the crying! You can read it over and over on here and many other forums.
If the chickens are coming from different places, than yes, you need to isolate them separately. Tend your current flock first, then the new birds and practice biosecurity to it's utmost! Also, keet the new birds, as far from your current flock, as humanly possible!
I personally would NEVER take in chickens from other than a reputable breeder or reputable hatchery as day olds...if problems arise, and they so often do, it is not just your flock, but others flocks you are risking. Diseases can be carried on a person, and we all go to the same places, the grocery, the feed store, ect ect. If you haven't already, you might want to do some reading on the emergency, disease, ect thread. It's so sad to read what others are and have gone through, by bringing in birds from unknown sources. You have a good heart, but there are just too many risks bringing in birds that may or may not be healthy. JMHO
