[RC-Monster Mike]

While there is no universal "right pinion for 5s", there may be a "right pinion for you". As you go higher in voltage with a given system, you want to go lower with the pinion ideally. What is "right" will ultimately depend on your particulars(terrain, driving style, tires, etc.), along with your intended purpose(bashing, speed runs, racing, etc.). If it performs as as desired and results in acceptable heat(or lack thereof), then it is "right" by most standards. :)