I agree, with 6s a 12t pinion is a good place to start. It will react a little slower to trigger commands making it less freaky, plenty of power, and plenty of kick for backflips. It won't be slow either.
Driving a 6s truck is a lot different than a 4s one. With 6s you do most of your driving in the middle of the trigger. The only time you bottom out the trigger is for stunts or maxing out a speed run. My recommendation is to run the truck on 6s till you get used to it. A Kraton is so much better...