, I think in your list of common gaits you forgot the fox trot.
To me, the difference between a gaited head bob and a lame head bob are very different. When a horse is lame they bob the head when the lame foot sets down. So it's always in time with one foot. When a gaited horse head bobs with it's gait, it is a up and down rhythm that is like an impulsion system for the gait. I can't explain quite why, but they don't look anything alike to me. I think if a person has any confusion about that it will go away when they ride one. I have a Fox Trotter and I can definitely tell when she is tender footed or hits a rock, vs. having a good, strong, head nod. It is not even close. A good head nod remind me of an oil rig pump going up and down....it's like a fulcrum that gives them their momentum.
I personally don't use special tack. I use a regular western saddle and that seems to work well with my Fox Trotter and her son. If I were filthy rich, sure, I would probably try different gaited saddles to see if they are all what they are cracked up to be, but that's not the case and my Quarter Horse saddles seem to work just fine. It's really about fitting the saddle to the horse, the same with any other breed of horse. A gaited saddle probably increases your chances of good fit, if your horse has a typical gaited shape as determined by the tree manufacturer, but that's about it. It is still all about getting a saddle that fits your individual horse.
I do agree they need room for their shoulders. I tried a narrowish western saddle on my Fox Trotter one time (she IS fairly narrow under her withers) and I couldn't get it off her quick enough. She was a stumbling mess with that saddle. I couldn't believe it made THAT much of a difference.
I can't speak for other breeds, but my Fox Trotter can fox trot all day. She LOVES to move out. Of course the faster she goes, the rougher she gets, so I try to keep her energy contained. I suspect she is close to a hard trot when she gets going too fast.
"They" say that the pacing type gaits are hard on a horse because the horse has to be in a hollow frame to do them, but I don't know much personally about that. My horse occasionally paces (when I'm trying to canter mainly) but that isn't her main gait. If I had a pacy type horse I would just make sure I mixed up my gaits and didn't ONLY ride at a pace (the stepping pace is actually nice and smooth).