posted 10-31-22 08:47 AM EDT (US)   
Well, after trying again recently, I figured out how to do the thing from that old thread.

https://aoe3.heavengames.com/cgi-bin/forums/display.cgi?action=ct&f=14,39438,,10

turns out the UnitType flags influence where the units will position themselves within a formation.

For example, setting the Musketeer from AbstractHeavyInfantry to AbstractLightInfantry will make them mix in with Crossbowmen and Skirmishers instead of sitting in front.