What ever you do, there will be fights when you combine the two groups. It's simply what chickens do. They need too hammer out the pecking order for the new group. Most people suggest not combining the new birds until they are similar in size. Adding when too small can cause excessive bullying and also doesn't give the newcomer much of a chance to become more than a bottom member.
When I added two girls to my flock it was a bit stressful (on me), but if you let them hammer out who is the head chicken, things quiet down after a few days. If you interfere, you just drag it out much much longer.
Of course, it's a chicken coop, not "Thunder dome". If you see lethal combat, or chickens on bungy cords wielding chain saws, you should step in.
Riki