It's a good idea in my opinion, because now he gets training from strong teachers (learning rokushiki and haki) and he can also see the bad side of marines (government) and pirates. So with that and his past it makes sense that he joins the revolutionary army.

I think in the later chapters of the manga, they mentioned when he became a yonko. I think it was 4 years before the story began

In the 5D's era you can draw when you go first