Enrolment

I would try looking at the enrollment plugins.  There is an auto - enroll plugin that would achieve your first request.  I would probably use a cohort or a separate category for your paid classes.  Create a cohort and enroll it in all your paid courses and then add the student to the cohort when they pay.  Or create a category for your paid classes, I think I saw a category enrollment plugin that would enroll students in the category full of courses.  There is probably a way to automate this (probably another enrollment plugin) but, to be honest, I have not done much with paid courses.