True! By giving Ferrothorn Iron Ball, you could powerful moves such as gyro ball. But, it wouldn't be necessary to give it to him, since he's already slow. You should try giving it to a Pokemon who can learn "Bestow". Therefore, you can pass the iron ball to your opponent, making him slower, and available to hit if it's a flying type Pokemon. Therefore, you can knock your Pokemon out much easier.
I think iron ball would work better against your opponent rather than yourself. Seeing that gyro ball is the only move, I wouldn't recommend it.
But, it's your choice! And I respect your decision!
Hope I helped!