I don't think the soviets would have responded if we nuked China. Stalin isn't stupid and the last thing he wants is another 50 million soviets dying. Although, the same could be said for America, which is ultimatley why we didn't attack.
But there is no way Stalin goes to war with the US over the nuking of China. In fact, he almost certainly would have invaded china himself to grab as much buffer room as he could before bumping up against UN forces.
China on the other hand would be pissed as all hell. A devestating nuclear attack against china would not have been the end of the chinese communist party, especially because the russians would fiercly and overtly back them against nationislt forces.
Ultimatley, instead of having a war against china over north korea, we would have a huge proxy war with the soviets over china. In the long run, that's not good for global stabilioty and makes it significantly more likely a hot war touches off eventually.