you can do facial animation either by using a rig with facial bones or by using blendshapes.
if you choose to use facial bones you should add them to the skeleton, yes.
you could make different rigs for different characters or make one shared skeleton with the same facial rig. this depends on how different your characters are from eachother (i.e. you probably want a different setup if you have a human and a werewolf or a xenomorph alien, but across diferent humans it’s ok to use the same one)
blendshapes is the most used method but I personally prefer a facial rig.
it’s in my plans to allow character customization of facial features, and that is best done with blendshapes. once I get that going, I doubt that using blendshapes also for animation will play along well. maybe it would work, but I don’t want to find out the hard way 