Well if mod developers want to have events focused on realistic formations, then there is no discussion about it.
Large regiments should be forced by rules to form a tercio, just like they are forced to form a line in NW. They can be line if traveling to another position.
Smaller one-class regiments can either form a line along side of tercio, or make smaller tercio anywhere next to big one.
Big tercio can be spread anything up to two man spacing (to allow helper troops on edges to enter the safety of their formation without too much pushing and squeezing)
If small regiment is making a line along side of big tercio, and their line is longer than the side of big tercio, they are allowed to make more than one rank. (Ideally, small musketeer regiment would make a line of two ranks along side of tercio, and would fire two volleys, one per each rank, one while the other is reloading.)
Some problems:
-People at wrong side of the tercio might die of boredom, as they spend most of time facing away from fighting and catching bullet drops.
Solution: more cavalry, let two cav per team be standard for events, so there is more flanking going on, so infantry can feel this tactics is serving a purpose rather than limiting
-Tercios are harder to form than lines. Actually it is surprisingly hard.
No solution here but regimental training. And a good system, like form a small line next to officer first and then keep adding lines behind. But still, it can be surprisingly hard, as a military formation in practice is only as agile and smart as the dumbest and clumsiest guy in it.
-Tercios are hard to move.
If tercio wants to move, walking (shift) is enforced by rules. Moving a formation is much easier with walking on, although it doesn't guarantee success it will prevent total breaking of formation on each move attempt. And of course, with drummer in ranks, walking in a formation is atmospheric as hell. Small regiment helper ranks are still allowed to run.
If big regiment wants to move by running, they would have to make a line first. Ofc, when melee is near or charging, running should be allowed.
Examples of allowed formations:
0 - big regiment muskets
1 - big regiment pikes
A - small regiment A
B - small regiment B
Basic tactics with small regiments covering sides of big tercio
B B B B B B
0 0 0 0 0 0 AA
0 1 1 1 1 0 AA
0 1 1 1 1 0 _ A
0 1 1 1 1 0 _ A
0 1 1 1 1 0 AA
0 0 0 0 0 0 AA
Small square on edge:
_ _ _ _ _ _ BBB
_ _ _ _ _ _ BBB
_ _ _ _ _ _ BBB
0 0 0 0 0 0 AA
0 1 1 1 1 0 AA
0 1 1 1 1 0 A
0 1 1 1 1 0 A
0 1 1 1 1 0 AA
0 0 0 0 0 0 AA
Big tercio is pikeman only, and spread out
_____________________ BBB
_____________________ BBB
_____________________ BBB
1 __ 1 __ 1 __ 1 __ 1 __ 1 A
_____________________ A
1 __ 1 __ 1 __ 1 __ 1 __ 1 A
_____________________ A
1 __ 1 __ 1 __ 1 __ 1 __ 1 A
_____________________ A
1 __ 1 __ 1 __ 1 __ 1 __ 1 A
_____________________ A
1 __ 1 __ 1 __ 1 __ 1 __ 1 A
_____________________ A
1 __ 1 __ 1 __ 1 __ 1 __ 1