Horizontally fixed? Yes in that they`re always the same distance apart. If I understand your concerns, there`s plenty of room between the two clamps for your stem, but they still fit quite well between the curves of your bars, so they don`t end up trying to clamp onto the curved parts.
Or is it the rotating part that throws you? In that respect, they don`t lock into place. You can rotate them up or down as needed (before you clamp them tight) and supposedly they won`t slip down (after you snug down the clamp bolts) with the weight of the bag. In practice, I`ve found that the Cdale mounts do indeed slip down with time, but that can be overcome easily. On one bike, I mounted a little aluminum "finger" to hold up the bottom of the mounting plate, on the others I found that putting a little piece of inner tube between the bar and the clamp gives it plenty of grip.
If you still have doubts, I suggest going to a bike shop that carries the bags you`re interrested in and ask them if you can try it out in the store and on your own bike to make sure everything is compatible. You won`t have the kind of selection that`s available over the internet, but you will be absolutely sure that the product you buy will be useable- that`s the advantage of buying stuff from your LBS. Honestly, I doubt you have anything to worry about- the boys at your bike shop will be able to fix you up with something and explain it all to you with real products in hand and with your own bike.