How does the AI pick army composition?

[konny]

I don't think so. In this case the armies would differ depending on who is the human player, what is - to my obeservations - not true. AS, for example, has always about the same army composition, regardless if I play the Ptolemaioi, Pontos or Armenia. When Rorarii were classified as "light infantry" the AI spammed stacks of them, since they are classified as "spearmen" they do disappear form AI SPQR armies as soon as the AI can field something better in the respective provinces.

In general there is a tendancy of what could be called "top or flop": either loads of elite units or masses of cheap peasants (sometimes mixed in a stack), omitting the "professional" class. For example when the AI is able to field Pantodapoi Phalangitai, Klerouchoi Phalangitai and Argyraspides from a single settlement it will most likely recruite either Pantodapoi Phalangitai or Argyraspides (or even both) but not the Klerouchoi, who would be most likely the backbone of a human raised army from the same settlement. When it recruites Klerouchoi Phalangitai it usualy isn't able to get Argyraspides and so goes for the best there is.