A useful tip to modders: as well as opening the file with your hex editor, also open it with notepad, which gives you a far greater feel for the general layout of the battle script.

I suspect that it shouldn't be too difficult to add extra units rather than just swap one for another, it's just that in a hex editor it's difficult to picture the overall structure of the file and thus important info gets left out leading to CTDs & the game refusing to play.

I think there are still limits though: I tried to swap the Roman's light cavalry for heavy and arcani for velites, getting a CTD with an error about missing graphics for both units, so I would guess that we only have a subset of available units rather than the full complement.