I have ridden my big horses/ponies in the bitless bridle and both the horse and I like it much better than a bitted bridle.
Also have used the driving one on the bigger horses.
However, just a word of warning, Dr Cooks bittless driving bridle for minis only fits small "A" sized minis.
Ordered one a few months ago, and it was wayyy to small for my minis head.
I really wanted one for Maggie.
So might be a good idea to measure your minis head, and ask the buyer to measure the bridle.
One thing I do to get the horse used to the idea of turning is....
Take a Halter, the ones that you can adjust the chin on. Lower the halter 3 flat fingers from the edge of the nostril. Tighten the chin strap to one fingers width. Clip driving reins on the sides of the halter, and start your mini that way, in a round pen or enclosed area.
They tend to get the idea faster using this method. There is also less stress or confusion.
Once your mini is going well in the halter, which will be a month or more depending on the horse.
Can get Bit ring clips, and attach the bit to the halter, adjusting for the best fit of the bit.
Still clipping the driving reins to the halter, let him just wear the bit for a month or so or until he stops fussing with it.
Then you can move on to clipping the driving reins to the bit. But if he has problems, take a step or two back and start over with the halter.
Training is rather like a dance, 2 steps forward and 1 step back.
Good luck!