Whenever you bring in new stock, especially when you don't really know where it's come from, you are best to quarantine them from your existing stock for up to three weeks. This gives you a chance to see if they have brought any health problems with them, before exposing your existing animals to contact with them.
As your birds have already mixed together you are too late for quarantining, but if you can put them somewhere where they can run beside your existing flock and all see each other for a couple of weeks things should go more smoothly when you finally put them together. The old birds will get used to the idea of the new hens, and the new ones will gain more confidence as they learn that this is their home.