Originally Posted by The_Mark
Yes, descr_formations.txt is the group formations, descr_formations_ai.txt is the army formations.
Find this line in descr_formations:
begin_formation ordered_single_line
Block 0 is the center block, 1 is right of center, 2 is left. Just change the unit types to the order you want, e.g. in block 1 add a line
unit_type heavy infantry 1.0
That will assign heavy infantry to block 1, to right of center. Remove the light and missile/skirmish unit type lines from block 1 and only heavy inf will get assigned to block 1.