As for the microcontroller. The size of the file you upload does not matter (as long as it fits in the space you have).
If you have an IF statement around the asynchronous code - the code does not run on non-asynchronous motors, if you say have config params that define the motor type your are controlling.
Thus, code inside the IF(asynchronous-motor) statements, never run, and thus never use CPU cycles and resources. (other than a single clock-cycle to check motortype in the IF-statement and some minimal amount of ram to store the motor-parameters.
Having an all-in-one solution, like Paul is doing, is the most flexible for the end-user
Edit: got some toys today
