From what I can tell, the most important combat buffs for the regiment in an engagement are the Major-Colonel, Musicians, Flag, Lieutenant, and NCO in that order. I recommend some experimentation to see how much of a bonus these buffs provide and whether it's worth making sure they're present during the event.
After some testing on the Sharps rifle, the bugle makes you reload about 2 seconds faster. The Major/Colonel gives you a noticeable reduction in sway while aiming down sights. The NCO gives you about 5-10 meters of extra sprint distance.
Upon further testing, the flag buffs are the most powerful that we have seen. They are primarily defensive buffs that reduce incoming damage from enemy stabs. We have found a noticeable difference in survivability when one side in a melee has a flag and the other doesn't. This also stacks with two flags. I recommend one flag per 10 soldiers in line because the flag provides 10% bonus to line survivability, equal to one person after 10 rankers. This is doubly important in all charge where hopefully the whole team will be together to get its buff. It does, however, need to be carried while it gives its buff. Sitting on the ground does not provide any extra survivability.