jpuke,
I took a look at my Sakos and Tikkas, and confirmed my general beliefs. The Tikka and Sako receivers are indeed different. Specifically, the upper part of the receiver where the rings can attach directly are different. On the Tikka, there is a notch on the portion of the ring that is closest to the barrel. On the Sako, the notch is on the portion of the ring that is closest to the butt of the gun. The rings have a "stop" that is extended below the rings and fit within the notch. As such, this type of system prevents the scope from slipping from the recoil.
I use Warne rings on my Tikka WHs, and they attache directly to the receiver as indicated above. On another WH I just noticed that I have another brand of rings that also attach directly to the receiver. They look like Opti-locks, but I'm not sure.
On my Sakos, I own 2, and only 1 has a scope mounted on it. It is a Sako 75 SS and that gun (if I remember correctly) has the opti-lock system that has both the rings and the bases. (The gun is at my brother's house in Virginia).
I am not familiar with the XS sights so I can't comment on them.
Hope this helps.
Zachary