Class choice in Incarna is a role-playing challenge more than anything. While any class is possible in the right situation/background, race, culture, gender, regional issues, and social norms all factor into what path a character is able to take. A full Character Background and Perspective Stories can explain anything, even complete deviation from any setting or rule restrictions. However, it is also these constraining elements that give the most role-play challenge (and reward) and should be encouraged. Multi-classing after character creation has only the limits the story and setting provide (aside from any normal Ability limits).