For pleasure riding and ranch work. And yes, only on a horse fitted for one. Hunters or jumpers accustomed to English saddles may not take kindly to the extra weight.
horses
Yes
They aren't. Arabs have one less rib than other breeds of horses but that is unrelated to western saddles.
Boots and Saddles
In the horse world, you can have many types of saddles, but the two basic saddles are English and western. There is a cross between the two of them, but not as much people ride in them. The western saddle is much bigger and heavier than the English one, and has a horn. The cross is shaped as a western saddle, but does not have a horn.
There actually are. Depending on what discipline you use, or what kind of competition you are in, specialized saddles and multipurpose saddles are used all the time, in both English and Western disciplines
Blazing saddles? Blazing Saddles
the ones that are tooled (decorated) are mostly for show. lots of leisure saddles have no tooling.
Bicycles', as in 'The bicycles' saddles were very worn.'
saddles western styles
Western rising saddles were developed from Spanish saddles brought to south America. The saddles found their way into Mexico where the cowboys there began changing them to suit their needs and eventually found it's way into North America.
You could probably find used Western saddles at a farm, or pawn shop. Pawn shops' might be a little bit cheaper on the saddles, because farm ranchers usually take care of their stuff better.