20 000 is more than enough for a civil war.
Only luck of HK is China has control above the sea, so the Capitol cant shift the situation to civil war by shipping heavy weapons to the rioters. (like in Syria)
You can't be serious...
First, 20,000 is enough? vs. PLA on home field?
Second, do you really think the US would be that daring as to instigate insurrection on a peer-level adversary? The potential blowback would be too heavy a price to pay.
Even if you look at the strategy in Eastern Europe, it is one of "containment", not direct confrontation, even when face to face like at Pristina Airport back in the day.
Think about it this way. LA Gangs in 90's got Norinco banned from the US market, that was not an organized governmental effort. If the US government were to arm HK insurgents, then those smuggling taps would turn wide open, and LAPD police stations would look like the scene from the original Terminator.