Since I just lost 1 of my 7 to disease and found out that of the 6 left, definitely 2 and possibly a third are roosters, I'm planning on getting another group of young chickens in a few weeks. Once I get them, how long should I wait before mixing the two groups? They will probably be 12 weeks or so old when I get them. My current flock is about 18-20 weeks old. I know I need to give them some time separated from the established flock so I can check for diseases and such, but is the age difference too much when they are only 3 months old?