'Musketeers' is for modding purpose. in the new game i don't really agree with this name being a unit name, as IRL, musketeers tend not to operate as homogeneous units (at least... not until 1660-1680s with first bayonets came to be).
the actual name is Pike and Shotte.
but if stackings (balanced) is to return, it is still possible to maintain separate Musketeers and Pikemen unit, wti the former is nerfed to be vulnerable to any cavalry charge and thus needs the latter to protect, and combat resolves should calculate as follows
Attacker (any cavalry)
Defender (a stack of musketeers and pikemen)
1. Cavalry charges
2. Musketeers fire some shots
3. Cavalry locked into a melee,
4. Pikemen blocked Cavalry attack, taking damage in place of musketeers while also inflicting significant damage to attackers