We have two 2-week old incubator hatched chicks in the brooder now. Today we acquired a hen with her four chicks that look to be about 2-weeks old. At what age will the incubator hatched chicks have sufficient immune system built up that we can put them with the natural hatched chicks and the hen? These are all of the chickens we have so I'm more concerned about disease for the incubated chicks, than another chicken picking them off - figuratively or literally. Thanks!