What do you mean by, "Finally when they do canter it's w/ hard hands and hard mouths to keep them from going to fast...?" When starting out, "too fast" isn't necessarily a bad thing. As long as they're not breaking into a gallop. Impulsion is a good thing and once they've mastered the transition into the canter and continuing to canter, then you can work on collection and change of tempo (faster or slower).
It sounds to me like the rider is maybe uncomfortable at the faster speeds.
And he/she is holding them back and confusing them (leading to the fast trot). Maybe try the canter on a smaller circle (half the arena or so) to naturally slow them down without cranking on their mouth.
I'm guessing since they're trained for driving, they listen well to verbal commands. I'd start off as GallopAway said by asking them to canter from the ground.
Of course, if the horses are truly out of control, that's another story. Are they bucking or rearing or tossing their heads?