I know next to nothing about western tack and fitting. I DO know that the horn goes in the front. *G*
My daughter got the complete package for her birthday - new horse, new saddle, new blanket, new bit.....you get the idea. Anyway, I've had her working this horse in the round pen, just walk-trot for now, so they get used to each other. I've been helping her with the tacking and untacking since she can't reach.
So now for the questions.

The horse is a little long in the back. My English all-purpose fits fine, and he seems comfortable. Daughter's western saddle seems to fit fairly well across the withers, no excessive pressure. But, with no rider in it, it's not in contact with his back in the rear at all. One of the issues the vet saw when he came out for the "post-purchase exam" was a tendency for the horse to park out when he was standing at rest, and said that could be due to the previous owners bouncing on his back instead of moving with him at the higher gaits. If I were going English and had the same picture, I'd shove a lollipop pad under, and just go with it. What do you do with a Western rig? I expect this horse to be packing my kiddo all over the west on trails and such, but I want him to be comfortable doing it!