Oh, the 8th Route Army got some level of American support during WW2, albeit highly limited. And the Americans were working on joint projects before Tiananmen, such as upgrades to the Type 59 (T-54) and J-8s (iirc).
I still highly favor the Chinese import of the Su-57 as a cheap, dedicated dogfighter / strike craft that can take attrition while the J-20 does hit and runs.
Given the limited production run, and that it is a heavyweight "stealth" fighter, the Su-57 is unlikely to be to be cheap.
Remember that the PAK-FA was estimated at $100M flyaway cost, without R&D expenses.
But let's say the Su-57 is only half the price of a J-20, what then?
I reckon you're still better off with more J-20s - because having numerical superiority means you can keep sniping away at opposing stealth fighters, whilst your opponent has run out of BVR missiles.
And there's also the existing (and even cheaper) fleet of J-10 and J-11s for last ditch attritional dogfighting.
Plus the future J-31 stealth fighter which is smaller than the J-20, and which should be cheaper as well.