[Pearls Before Swine]

The Russians and the Chinese are not natural allies. Russia's population is declining, and it has continual friction with a number of the ex-USSR Muslim countries. China is growing in strength, and would like to reoccupy some of resource rich, sparsely populated Siberia. Isn't that obvious?

[Ahban]

Yes, in the long term. In the short term an ambitious China could "rent" an air force and an amphibious task force from the Russians. Russia cannot maintain this equipment, or pay its troops, so why not rent them out?

This longshot is the only near-term way I see that Tawian needs to worry about an invasion from China.

[batter]

Thanks for the bump.
Pearls is correct. Russia and China may be moving closer to gether, but Russian may only be using China while it's convenient. The two are also potentially heading for a steel trade spat:
China stiffens opposition to Russian steel
Russia-China trade disputes coming to a boil

Both found via ChiCom Watch.