VEX Motors

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),
…]