lol there is no way alexander can expect to get help from any ofthe state. it might be the case that some of the state might free ride the collective defence that everyone is bound to be part of, but no one is gonna openly side with alexander. if you are familiar with history of that time you should know how Chinese think of barbarians, there were instances where all the states who were at war with each other came to the rescue of states under attack from barbarians.
and if alexander were to march east he will encounter Qin first, then prolly Chu, which was the largest and extremely powerful state. i am not sure if Jin was still around by that time, but even if it wasnt, after it had dissolved one of the state was the strongest of all states at the time and that was Wei. Wei had heavy infantry as well like alexander. and Zhao has really good calvary. Han was less powerful but would have enough soldiers to slow them down. even if they were defeated, the weakened alexander would then have to face the most powerful state Qi. Qi's capital linzi was the largest city in the world til its fall, just to give you a gauge of how powerful it was. so conclusion, doesnt matter how alexander wants to go about it, you can give him twice the original number of soldiers he had, he will fail.
Actually in my post, I said that if we forcefully tried to take Alexandra vs Xiang Yu and/or Liu Bang (although they are from different era), there will be two outcomes for each circumstances. Although one of the outcome for each circumstances are highly unlikely (the joining of force for Alexandra and Xiang Yu or Liu Bang), and I am familiar with chinese culture (being one myself), but you must also admit that Chinese are a very flexible people. The reason why they do not think highly of the barbarians are not out of national pride, but rather, they looked down on barbarians.
The chinese had the most developed culture in the region during that period and nothing came close to their development. So they basically looked down upon all the rest of the people - even the Qin when they are in East and not yet entering the mainland.
Even the Yue from the mainland was also being looked down upon as barbarians.
However if a superior force establish themselves on the plain and managed to destroy or defeat any of the states readily, I believe many of the weaker states will align themselves with this force for many reasons (one of them being - to safeguard their own interest and to expand their interest base.)
Thus it really doesn't matter who enter China, be it the Romans, the Greeks or Qin, they have the same outcome.
But I am not saying that Alexandra will win. On the contrary, I did pointed out the disadvantage of Alexandra's troop when entering the Chinese plain. They are not just a measure of the capability of the troops, rather we must look into many other areas such as terrain, geography, supply chain, logistical support, etc.