You need a real bridle and bit. Sidepull does not work like a bit, the angle of the reins is different than a ridden horse, so driven animal gets signals differently.
As for "forcing" a mini to be obedient, that is a joke!! A LOT of folks think that way and have been hurt trying to "fix" the problem the lack of training of equine has put them into. Minis can APPEAR to be going along with the program, but have little to no depth in their training. When they reach their point of "brain overload" they have an accident or wreck. Even LARGE, STRONG people can't hold them if the mini decided to leave. I hear it over and over, "how good Fluffy was, got him driving so easy, only took a week or two!!" Then Fluffy ran away, reared up, threw out the driver and wrecked the cart. Now he fights when they try to drive. This is even LITTLE equines, they are AMAZINGLY strong, CAN drag you all over if they like.
Sidepull is designed for RIDER to put hand outward to pull sideways, to bend the equine for turns. Driving you can't put a hand out sideways from equine head, for that sideways pull, so the "tool" is rendered rather ineffective, maybe not even workable in stopping. With the sideways pull of rider, they can actually bend the head and get the horse going in circles. Driving reins are made to run thru the rein rings on the harness saddle, NO WAY to pull OUTWARD sideways.
Some tools can transfer from one discipline to another, but Sidepull is not one of them. Of course you will hear from folks who use them and "have never had a problem" while driving. Your choice. But if the situation gets sticky out driving, I think that Sidepull would be rather useless to aid you in control. Probably couldn't even stop the pony if he wasn't cooperative. Minis are not foolproof in every situation.