I am not an expert at this but have several minis that I am training to drive all are different. Mine with the willingness to work are progressing faster than my strong minded ones.( I know Chief will be fancy in the harness but man he is a handful! )
With riding horses I always would take a horse that was willing over a fancy show horse that had a nasty attitude towards work! I could make the homeliest horse preform beautiful dressage all because they wanted to learn!
We have a pony that I have "retired" from driving ( and riding for that matter) because of his nasty habit of kicking when handling his hind feet. I am sure someone could work with it but I don't care to take the risk of getting kicked and he makes a wonderful companion pony
Sum it up
Go attitude, sound, reasonable height for the minis ( I have a 29" that is as small as I am willing to drive) the rest is up to the training
I would add it depends on your goals. Of course great conformation and strong gaits will have more potential to progress physically, but I still say pretty is as pretty does. Need to have a good mind. Ok that's all I got