Originally Posted by Bohh
No you wont. If you've been using the 15C without problem thus far the 20C will be fine. Basically the C rating determines how many amps you can pull from the battery. 20C allows you to pull more amps (120amps)than 15C (90 amps) so if you're staying in the safe range for 15C (<90Amps) you'll be in the safe range for 20C. However, if you're running over the 15C rating you will run into trouble.
When you're mixing and matching parts such as you want to do just be sure to keep the amps lower than the lowest rating in your setup and you'll be fine.
I think he meant running them in series, I wouldn't. The differing c ratings mean they are different, even if from same manufacturer. And using different cells in series is almost always a no-no
sorry didn't read post through, My bad