How much is Khrushchev or Brezhnev going to launch? 50? 100? 500? China is a large nation, no matter how much he throws at China (keep in mind one nuke at China is one less against the US) he will not destroy it, even more so unlike the USSR or USA, China is at the time had 80-90% of it's population living in the countryside, so even if the cities are destroyed, the nation will go on.
The USSR had 1,800 warheads at the time, so let's say that it launches 500-600. The fallout alone will kill tens of millions of people in the countryside. Direct detonations in major cities will kill, as you mentioned, about 10-20% of China's population.
So, just to recap, we're looking at:
-Hundreds of millions dead (200-300 million doesn't seem unreasonable)
-Total destruction of all infrastructure
-Total destruction of all industry
-Long term radiation which will kill millions more
-Irradiated crops, which lead to famines, which will kill even more people
I'm not saying that all Chinese people would cease to exist. I'm just saying that the PRC government would be completely obliterated, and that the nation of China itself would take many decades, if not a century, to fully recover from the war.
And this is assuming that the USSR doesn't attempt to balkanize the country or fund local warlords after the war ends.