I have an off-track standardbred who came off the track as a pacer. His pace is really too rough to sit.
My rule for myself has always been to sit the trot (old dressage rider) until there was so much suspension that I couldn't sit, then the horse produced the post with his own impulsion. I would never post to make a horse trot EXCEPT - isn't there often an exception?- I've discovered that I can get Twist to switch to a trot if I post to the pace. A couple of trainers were working near where I was schooling him yesterday and I finally stopped to talk to one gal because she had such a quizzical look on her face. I asked her what breed she thought he was. "Well," she said, "I thought he was a quarter horse until I saw him move. Tennessee Walker?"