This idea, the 'universal warrior' is nonsense. Heck, the Macedonian phalangite wouldn't have fit in a Roman legion (Plb. 18.28ff) and vice versa. A Spartan wouldn't have been properly drilled to fight in either! War is not constant, and neither are combatants.https://twitter.com/SPressfield/status/1346185128742014976 …
They would not 'fit right in' even among each other, even in the most limited sense of being able to function in the formation. I think Polybius speaks to how Pressfield here fails even at the mere pons asinorum, much less the more detailed point.