Will horse shoes effect my horses extension...?

In general, if a horse has lovely movement without shoes, they will have a lovely movement with shoes. Shoes tend to affect the mediocre movers more, and they can be more affected by the weight of the shoe. The lighter the shoe, the less knee action, which is why you see a lot of western pleasure and hunters in aluminum shoes. Gaited horses tend to wear the heaviest of shoes. Please use a farrier that does have some experience, and not the cheapest. You may find that your horse actually improves with shoes if it relieves pain. He may be more forward and actually have a longer stride. "Mushroom" feet are some of the most difficult to deal with. They do much better in a dry environment where the sole can toughen up, because the sole ends up bearing more weight than the average hoof because of its proximity to the ground. Wet weather and muddy turn outs are not their friend. It will make them even more prone to spreading, bruising and abscesses. A farrier or trimmer can do nothing to "correct" genetics or environment. They can only support hoof health/balance to make your horse as comfortable and usable as possible.