I think it's this part in the scenario.ini
[rank]
cmdlvl1=2
cmdlvl2=1
cmdlvl3=0
cmdlvl4=0
cmdlvl5=0
cmdlvl6=0
cmdlvl1= 1 for FRENCH, 2 for ALLIES (I'm not sure if this is always like this though, it might reverse if you build your own OOB with allies first).
cmdlvl2= Army #
cmdlvl3= Corps #
cmdlvl4= Division #
cmdlvl5= Brigade #
cmdlvl6= unit #
Best to check the scenario.csv which contains these indexes for each unit in the game, find the one you want to control in the scenario and put those values in the section of the scenario.ini that I mentioned above.