With saddle fit, whatever is comfortable and fits your horse and you. Others have written plenty of good advice.
Bits - When it comes to gaited horses again, what works. What I have found is that curb bits help on gaited horses so don't be scared to try different ones. Those who go on about curb bits not being useful on gaited probably havn't ridden gated horses much. I ride my QH cutting/cowhorse horse out on the ranch on a loooooose rein, even at a flat gallop. I mostly use leg cues for direction. I also ride my TWH on the farm on a loose rein, at the canter or dog walk, and neck rein, and use leg cues the same on them. BUT if I want to gait well, I take up contact (on a curb bit) and hold my horse in frame to get the best flat walk, run walk or rack that that horse can do. Perhaps due to conformation imperfections they really need help with self carriage, ie they don't have it. The position of the back influences the quality and rhythm and steadiness of the gait. Particularly walking horses which have so many gears and so many gait options at any one moment some work must be done to put them just into one. The curb helps you communicate that, yes, I do want this amount of back flexion so you stay in a runningwalk both up and down hill when the horse would naturally become more rounded and trot uphill and hollow and rack or pace downhill. So I gait around with contact and need to constantly adjust to get a good gait and drop the reins and canter off or slow to a dog walk on a cutting horse type rein.
My TWH also does reining and is very responsive and neck reins - but I still need contact on a curb bit to get good gait and not a dogs breakfast.
I can see exactly why at a fast rack and particularly a speed rack contact is probably almost always essential. The same reason an overcheck rein up the front of the face, between the ears, to the saddle is used on harness racing horses, for them to lean on. Unlike classical dressage the horses back is flexed the other way at a speed rack with the nose forward and the back dorsiflexed (or is it ventroflexed?) or at least level back.
I can see exactly what the guys riding racking horses are on about when they talk about curb bits.