If I remember the way I did it is quite simple for ECW.

Combined Company's can hedgehog as a single unit - without with any other units. So they just hedgehog as a single group, as they already are a combination of troops. I did not allow any other units to also join in - but you could allow another musket unit to join if you wanted, I guess. This then is just a different formation for the unit if you think of it that way. Note, I just marked that on units - as it's hard to show it any other way. As always it's a move or command reaction to change to this formation.
If a unit is in the hedgehog formation, then all the normal rules apply - they can't move, have no flanks rear and have a shooting value of 1 all around them. They also need a command test & move to change back to normal. If 'changing formation' to a hedgehog as a charge reaction fails then, it does not get to reaction fire, but still would get the advantage of being pikes.
Using the rules this way the hedgehog is great at making combined units pretty impervious to charging cav or rear attack but makes them very static - so vulnerable to just being shoot to bits by combined arms forces.