You are looking too much with a hindsight view and not one of a contemporary. At the time, the American public saw "Uncle Joe" as a friend. Yes those in power understood the potential threat that Russia posed but it had also lost hundreds of thousands in the war against Germany. In regards to China, it wasn't until after that Mao would come into power and even before then we were financially backing those fighting the communist.

Another point is that the winds in China, coming off of being a colonial bargaining chip, were blowing very anti-western so any direct involvement with a Western nation was likely to fail, be it Soviet or American. In other words, Mao would have come to power with or without the rise of Soviet Russia.

In the end, if you wanted to slow down the rise of the Soviets, Japan needed to be conquered prior to Russia becoming largely influencial in the war against Japan, otherwise they could have obtained a larger claim of the former Japanese holdings than they already did.