Regarding the pinion clearence, one of the biggest issues using mechanical brakes is that because of the motor can hitting the disc there is a limit on how small you can go on the pinion. I solved this by spacing the motor back a few milimeters so that it clears the disc and therefore I can run very small pinions (10, 11, 12, 13T). Since I am running the Kyosho 46T spur and a hot running Feigao I need to run a 12 or 13T pinion. This also eliminates clogging. On the downside, this setup puts the pinion on the very end of the motor shaft. Because of this I built up the motor mount/chassis to be extra stiff (no flex). So far I have had zero issues with this setup.
This is a 13T pinion.
These pictures show the test fitting and the rough parts (motor mount).