I fear that Zhou as itself at 550BC was rather weak. It had lost considerable control over the feudal states.
The Zhou state was weak, but the Jin wasn't. China has been under the hegemony of the Jin state for most of the 5th century. All the major states, such as Qi, Song, Deng, and Lu were under the control of the Jin at this time. So China was not disunited, although far from centralized control.
I'm not sure what the thread is trying to answer, an invasion of China by Cyrus and vice versa or merely a military comparison? The former is virtually impossible at this time.