Because established canon directly contradicts the fanon, you cannot use the fanon ending. Please stick to canon events (the person is not Zechs, Relena and Heero are poisoned and enter cryostatis, etc.) if you wish to take Wufei from after those events occur (aka post-Preventer 5). Since these events affect any potential future castmates as well, canon events must be followed.

If you are uncomfortable playing a minor, there are options to age the character up without the canon knowledge in the game.