Do i need to pick a subclass for ALL my characters right now in order to progress at the Sentinel Village or can i pick some later?
I am thinking to wait until i unlock those last 2 classes (Bushi and Imperial i think) to sub some of my characters, or can i change the subclass at any time later?
You can pick them at any time. If you don't want to subclass some of your characters nothing should keep you from doing so.
You can reset the subclass by resting (and thus losing 2 levels, a pittance). Feel free to experiment for the subclasses.