The magic controllers and internal controller wont melt your motor. But if you want to run a single MP motor with it's full power you need to go up to a 12 fet controller.
The on resistance of a 6 fet controller is going to be lacking in the phase currents department.
A full modded internal controller, got me 50 amps at mid RPM slow off the mark, but 50 amps on a 12 fet controller almost flips the bike upside down if you don't lean forward on the take offs or program it properly, and a 18 fet or 24 fet controller will burn your motor out easliy if youre not careful.
The internal controllers are perfect starting point for people who are new to Ebiking
The internal controllers would have struggle hard to hurt most good batteries and would fail to burn an MP's windings..
It is a lot of hard work and an one requires to master the learning curb to get the best out of these MP motors.
The 20' should be a good performer on the 6 fet controller. Some are having a few power adjustment problems with the torque on a 20" Pie using 12 fet controllers, too much torque and they are almost falling off their 20" bikes using a touchy throttle that springs the bike off the mark. The six fet internal controller should strike a good balance of power and speed for the smaller rims.