If you haven't given up on us already ... I just want to welcome you to the forum.
I would be starting your colt out right now with desensitizing to the world in general. Teaching him that whoa means "stand still until I say other wise" and other basic in hand manners will be time well spent when it comes time to put him to. Take him for walks where he can experience all the world will surprise him with, cars, dogs, birds flying up and anything else you can find. At this age just getting him out and about will help him prepare for his future job as a driving horse. I would start ground driving after he turns 2 assuming he is understanding his in hand work well and once he is able to give consistent correct responses to rein cues and whoa while being ground driven then I would be driving him in all the places I took him for walks in hand previously. Not until I was confidant he was going well there would I even consider introducing him to the cart. Just how I would do it and I'm sure others will chime in soon with other suggestions.
As for the mare, always assume (for your safety and the horse's) that any training you haven't seen demonstrated is exaggerated and start from the ground. If your mare is trained you should be able to move thro the steps of in hand work- intro to the harness- ground driving- intro to the cart- and finally hitching and driving, fairly quickly but be prepared to find out she knows far less than you had hoped. Stating from the beginning will also let you and her learn each other before actually driving her.
