heavy cavalry engaging heavy cavalry

[Macedon]

Well, that is one source. Yet isn't it interesting that your clearest source is Machiavelli, who aside from one attempt to reform the militia of Florence was a book soldier? Using him as a source on early 16th century warfare is like using J.F.C. Fuller for early 20th century warfare ... he can be valuable, but not reliable.

If anyone has sources, I would be happy to see them in another thread.

Machiavelli was surely much more knowledgeable in these matters than you and me combined. He wrote for people who had vastly more military experience than his and had a reputation to defend. The epitome of a pragmatist, he would not have written something that at least was not generally recognized as common military truth. He is very reliable in the context of his era. However, if you would like to delve more into it, please make (or resurrect) another thread.

Btw, could you give a ref regarding the Bactrians and the Thessalians? I do not seem to be able to find it in my translation of Rufus.