Here's my suggestion, which is what I think I'm going to use on the setting I'm writing at present:
Common Magic is a restricted skill, so you have to join organizations to cast them. So, Common Magic is taught by sorcerous organizations such a Wizard Guilds, Witches Covens, Changeling Cults etc. for your examples. Since you want use to be connected with a stigma, apply a penalty for cult membership: Likely social ostracizm at a minimum, membership possibly punishable by death if the stigma is at inquisition-era Europa levels.
This fits in with the Legend emphasis on Cult membership (which you may or may not use for yourself), and then Trolddom and Sortekunst (Hej från söder!
) are taught only by cults that are proscribed or outcast.
If what you are saying is that you don't want any magic user to start off too powerful, then stick with the decimal rule you found, it seems like a good solution without adding too complex extra rules.