I don't think it works that way. I think the lines like:

16 MTW2_Fast_Musket

are the animations that the soldier model uses but the model itself has the weapons it uses. So the musketeer model doesn't have a pike so it can't use those animations and the pikeman model doesn't have a musket so it can't use the musket ones.

I think the only way to do it would be with the formations file trying to get a formation where the musketeers and pikes are formed up as a group. Never edited the formations file though so don't know if possible.