Quote Originally Posted by Goblin_Priest View Post
There's no rules against multiclassing into PC classes. As the joke went in the early strips, regarding Elan ditching all the education of starting as a wizard by just multiclassing into it. Rogue, barbarian, bard, cleric, and/or wizard would all be pretty good choices for a farmer, for example, or just about any professional. Any class, including the other NPC classes, would, actually.

A lvl 20 commoner is absurd. Commoner is basically the class for the untrained. An epic level commoner is basically a godly trained untrained person, who is godly good at absolutely nothing.
Sure, there's no rule, but the average peasant isn't going to get rogue or barbarian training. The average peasant is going to work on his land all of his life. He doesn't have opportunities to take a different class.
At best, he gets other NPC classes, such as expert if he manages to get a trade of his own or warrior if he joins the militia.