"Russia's got the nukes and weapons development going for them, however the country's economy is terrible even though they are the 2nd largest."
Russia's economy is far more resilient than people realize. Remember the sanctions that the Obama Administration put on Russia as did the EU? Those were suppose to crash Russia's economy. Russia has not crashed yet.
"To get into a large full scale conventional war, you need the a good support base with technology, money, manufacturing, skilled workers, allied support, etc which Russia doesn't have."
I don't think Russia is at such a disadvantage as you speculate. Don't forget, NASA still is dependent on Russia to get into space these days.
"China has the manpower, growing economy, and manufacturing going for it. If both countries allied and built up their defenses in a few decades, then it would be a brutal WWIII."
Should those two countries militarily ally themselves against the US, America does not stand a chance conventionally, and most of the World would be uninhabitable if the war goes nuclear.
Kuleana