4 legged mules

Mules are made a little different than horses and there are saddles built on Mule Bars, if you were just going to ride a mule that would probably be the way to go, mule bars are straighter or don't have as much "rock" in them but you can also get along with a saddle built on Quarter Horse Bars, typically it would be one that is not super wide in the Gullet. Whatever you ride especially if you ride long distances or are really working your mount you need to pad them correctly, that varies greatly from animal to animal and there lots of ways to achieve that. It doesn't hurt a thing and could help some mules or horses to cut a 2-3 inch gap in your pad from right behind the withers to a few inches from the back of the pad, the idea behind that is to not have pressure or rub right on the spine, but if the saddle tree is reasonably close to the horse or mules shape it is not absolutely necessary. There are lots of elements that go into achieving a good fit of a saddle and pads if you get right down to the nitty gritty and we can go way overboard with all that.
I have never owned a saddle built on mule bars "but" I think someone just going to ride a mule and inexperienced in saddle fitting would probably be ahead of the game to purchase a Corriente brand saddle built on mule bars, they are a fairly cheap saddle of reasonable quality that you could probably get years of service out of.
Have never been associated with Tough 1 saddles so don't have an opinion on them.