Great questions! Size sometimes matters, sometimes doesn't. I have a flock of 5 Barred Rocks, and 2 Sex-Links (Red and Black), and I was worried about moving in my Leghorns because they're smaller. However, it was a great transition, almost immediate, so I think you'll be fine, though if you have any cocks, there could be trouble with dominance.
To mix, put new chickens in a separate, yet adjacent pen from the others, ensuring they can still see each other. Make sure the top is covered so they can't get in or out. After a few weeks, or until they get used to each other, you can try having them together. If the reaction is too aggressive, then try again later.
I'm not sure if it might be different because of the number difference, but PLEASE tell me how it turns out, because I may be adopting a couple of pullets, and am worried a little about my 13 vs. 2.