I'd recommend just using C20, at least for a "vanilla" C20 game. It's much better written IMO than M20, and is a lot more straightforward. There's just a lot of stuff from various supplements packed in that makes it so big.

As to your Mage background, don't try to read Changeling from the perspective of Mage. Lots of things will get confusing, like you probably thinking that the Dreaming is an Umbral realm, while not actually following the rules of Umbral realms. Crossover is certainly possible, but you have to have both games stand on their own for their respective dom...