This topic came up on another thread and I did not want to take away from the original poster, but wanted to answer some questions. One thing I have learned is that there are no absolutes with horses, so it is always best to work with a knowledgeable fitter in person. These are just some ideas on fitting the Spanish type horses.
I am going to be attaching pictures of my sister’s horse who is the bane of my saddle fitting world. One of the hardest thing with him and a lot of Spanish type horses is getting the saddle stable, but not inhibiting their movement. In general, they tend to be very board in their spinal processes and need a lot of space between the rails and at the head of the tree. Some are flat and some are actually deceivingly curvy.
Looking at him, he is appears to be a great horse for a hoop tree. However with a hoop tree, the saddle is even more unstable than normally. He is very sensitive and hates any extra movement of the saddle or rider. Because hoop trees tend to have shorter tree points, it does not support the front of the saddle enough for him. This can cause extra moment of the saddle.
He is also very mobile in his back and throws the saddle and rider around at every opportunity. The longer tree points in a saddle helps the saddle stay in place better. Having said this, he does do well in a saddle with a more open head (u-shaped) with space between the rails. Just not technically a hoop tree.
I hope this helps and let me know if you have any questions.