Just an update:
I went ahead and bought two new pullets that were approximately the same age as our original two. We introduced them in their new outdoor run all at the same time and everything went swimmingly. A few pecks, a few flapped wings and everybody is getting along fine.
I'm in a similar situation. I'd like to pick up two more chickens to add to my two 6 week old pullets. I can either pick up chickens of a similar age or go with chicks and raise them in a brooder separately. We just finished their outdoor housing this weekend, so we're planning on moving the...