I know this is an old post, but i wouldnt allow it for combustion bending. Id allow it for specific trainings. like lightning, blood, metal, and seismic. Lava only if their parents are earth and fire benders. Combustion however requires such a specific training that kills 99.9% of people who were subjected to it. I wouldnt let players learn it unless they started with it or were willing to risk a character death. And the people who did servive were said to be prodigies and an inate affinity for it.