Does anyone know if it is legal to use different VEX motors?
They’ll be similar but just a little stronger?
Would that be legal?
Nope.
The only permitted extra motors are:
<R59> Additional motors specifically permitted on 2008 FRC ROBOTS include:
…]
FIRST Tech Challenge (FTC) servos (Innovation First part number 276-2162),
FIRST Tech Challenge (FTC) motors (Innovation First part number 276-2163),
…]