- How are the units organized on each stack?
There is a hierarchy, the spearmen always will be put in the front line, and archers in the back by e.g
Your bonuses will depend on what kind of battalions class you have for each unit group, like spearmen + archers by e.g, each battalion class have their own attributes with bonuses and debuff.
- Is there a limit to the number of different subunits that can go in a group?
In summary:
Unit group = 64 units, that means = 16 differents battalions. Each battalion = 4 units from a class-specific (like spearmen).