What is the best way to introduce a new chicken to your flock?

I have 11 girls and 1 roo (All Amberlinks) they are 16 weeks old. I have been thinking of getting 1 or 2 new chickens, love the looks of Silkies. I am concerned with how to keep them seperate for 2 -3 weeks? I only have one coop and 1 run. I suppose i could keep her/them in an old dog crate, but it is pretty hot/humid here still im afraid they would sweat to death in there.

Any thoughts???