> Do you think Russia would use nuclear weapons against China if there was a small scale clash on the Siberian border?
I think it is impossible to have "small scale clash for territory" with nuclear state. They just can increase forces until nuclears can be applied by the military doctrine. So, nobody is interested in it.
P.S.
By the way, Russia and China adjusted borders couple years ago. By peacefull process of course and according to interests of each and other.