Hi TMPikachu
You forget that when Hannibal used his elaphants against the Roman legions, their commander told them to change formation to a loose formation so that the elaphants would just pass through them. The Romans where very adaptable and the close formation was what gave them the advantage over the loose formations of the barbarians. Order, discipline and adaptability is what seperated them from the barabarians (and even the professional armies they fought).
If the Romans saw that a close formation (or any formation) was not working they would simply order them to change formation and they would.
The Roman army was very flexible and trained their troops constantly in changing formations as soon as possible. They also trained with shields and weapons that were double that of the normal weapons and they received training from gladiator trainers and based their weapons training on that of the gladiator schools.
I also believe that Roman metals were good enough, and the lorica segmentata seems to be quite flexible.
And we mustn't forget that Syrians, Africans, Armenians, Jews, Iberians, some barbarians and all the nations and races under the Roman Empire from 212 AD were all Roman citizens. So to use any of them in battle woud not make a difference.
And don't forget that if the Romans see that they aren't going to stand a chance against crossbow attacks they aren't going to carry on marching moronically into a hail of arrows/bolts. They had their own archers and horse archers that had excellent bows and would surely be used against the crossbows.
The Romans also developed their own form of cataphracts from the Parthians/Persians, so they would have heavy cavalry available.
Hi Zuwairi
Yes, I have read the Art of War, the Wordsworth edition, which also has a commentary by a Chinese General from Mao's time.