16 no
18 or 19 yes but 5s will be way to much for inside clay unless its pretty big
just go by the temps
mine is geared 20 running 4s and kit seems ok motor does not get that hot I think 150 on the motor is good but not much higher
just ask some 1/8 scale guys at your track what they are running and stay around what they are running + or - a few teeth
another factor depends on how hard you are pushing the car harder throttle bursts make more heat