When I get home I will get you some good reading and links.
Short answer on soundness, no. One of my favorite three gaited horses was sadly retired last year at 19. Still rockin'! They are successfully bred to pick up their feet high, and do it well. You cannot force them to. Their soundness is essential to achieving that high action. At the trot, two feet are hitting the ground and supporting the horse at all times, and with the higher elevation the feet hit the ground with more force. If the horse is off, even a little bit, they will lose "motion". There will be unevenness in the gait, a head nod, and they will not pick their feet up any higher than necessary.
They are the BEST breed when you want that "buddy". They are bred for their inquisitive and personable nature. There are some that are nasty, but it is *usually* man-made. I grew up with them, so I am very biased, but I haven't met any other breed that can compare. I love other breeds, but they are just missing that...something. Hah, I love them.
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