Ok, I think I've found a .txt file for that (descr_formations.txt or something), but I have NO idea how to make a formation for heavy infs on flanks and light in the middle. I'd like to override the formation called single sorted line with my formation.
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.