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).
This page contains modifications to the iDnD core classes for using in the Incarna variant. For the most part, players can consider ©Players Handbook as canon – these modifications mostly consist of additional options.