Be very, very careful doing that or you'll get a crash course in other things you
really don't want to know about. If you put birds together from different sources, the new ones can be respiratory disease carriers (or the original ones could, really) and infect the others. You cannot always tell until there is a stress applied and BAM! Symptoms show up. Most of those diseases never leave a chicken's body-they are "Typhoid Marys" in the flock and can infect others even if they are currently asymptomatic.
Check out my thread here for details.
Urgent Reminder-Always Quarantine Newly Acquired Birds
Chickens are fun, but disease can really take some of the fun out of that. You can't prevent the reproductive stuff other than buying stock not generally prone to it, however, the respiratory diseases can generally be avoided if you use extreme caution.