You need to measure your pony and the cart and make sure it fits the pony. The American Driving Society and other websites list the measurements and how to take them. The cart needs to fit the pony.
The cart needs to be inspected for damage to the frame, rusting, bent frames, etc. All the spokes on the wheels should already be tightened to the same tension, the wheel rims should not be crooked or dented or rusted.
Much the same goes for the harness as well as how the pony is trained to pull the cart, to stop, turn, etc.
Most people don't feel driving is a big deal, or that a harness malfunction or problem with how a cart fits or how carefully the pony is trained, could cause any problem, especially with a mini, after all a mini is small, right.
No it still weights a couple hundred pounds and can hurt you, himself and others if the cart is not right or he's not trained right.
This is still possible -
here's one about not taking time in training:
Not saying don't do it - I am saying do it carefully.