I am a huge fan of the thinline and have actually taken the contour pad and cut it so it fits just inside of the Mattes pad that I use on a tb who goes in a saddle that is on the wide-side for him. The contour pad fits underneath all my jump and dressage saddles, even now that I've taken the scissors to it. I have both the regular and the ultra pads. Swear by them.
Love my thin line. I use the Ultra thin line half pad on top of just a regular quilted cotton pad for dressage. It gives protection but without the bulk of thicker pads. I tried thicker pads but felt like I was being lauched on the "up" moment of the trot.
I used to use the contour pad. Yes, it fits both saddles and no, you don't have to have a half pad. Mine lives in the trailer now though -- got out performed a hundred fold by my Ecogold half pad. But I did use the thinline for about 3 years with my older horse -- in retrospect, I don't think it did too much for him, although it did help my arthritic lower back a little.
I have the Trifecta and love it. I needed just a tiny bit of shim under my saddle as I have a young horse who is developing and the Trifecta is perfect....not a ton of fill like a big fluffy sheepskin, instead just enough with the excellent benefits of the Thinline. One of the most versatile half pads I've ever used, as I often find that a sheepskin is just too much.