I didn't start the game as an Immaculate Monk, but I want to learn Celestial Martial Arts!

Sure. Rather than deal with the many-year training process, there's a 6 xp surcharge on your first Enlightenment charm. 

That's not fair!

It keeps things even so that you don't get more from being a CMA-learner post-chargen as opposed to one in chargen, which would be unfair for those who start play as Immaculates.

Where did you get 6 xp from?

A secular dragonblooded starts with 7 charms. Assuming they are all favored martial arts charms, they're worth 8 xp. (Everyone uses Solar XP costs here.) That's a total of 56 xp.

If he's an immaculate monk, he starts with 5 charms. Assuming they're all favored martial arts charms, they're worth 10 xp. (Terrestrials pay 2 xp extra for CMA, however.) That's a total of 50 xp.

The difference, 6 xp, is how I got that number.

Don't Immaculates start with the two Enlightenment charms automatically?

I do not consider that to be the case, and yes, I know that John Chambers rules differently, but here I don't go with that ruling.

Hey, wait a second. The first two charms, the Enlightenment Charms, are by definition Terrestrial Martial Art charms. So they only cost 8 xp each, which is 2 xp less. Shouldn't the surcharge be 10 xp, not 6 xp?

A significant paradox! We resolve it by declaring that purchasing the charms in the two-charm Enlightenment trees costs the same as buying a Celestial Martial Art Charm, even though they are technically Terrestrial Martial Art Charms. Why? Well, they're very complicated (like Celestial Martial Art Charms) but don't require the purity of essence that Enlightenment requires.

Can I, as a not-Dragonblooded, get the super cool bonuses Immaculates get when using their form weapons, like the Speed reduction from Water Dragon Style?

No; these bonuses are from the Dragonblooded's elemental nature finding it's most pure expression.

I'm a Dragon-blooded, and and get the super cool bonuses Immaculates get when using their form weapons from one Immaculate Style.  Can I learn another Style and gain it's benefits as well?

No; you gain the bonuses from the Style matching your Aspect.  This is explicitly for balance purposes, and because the book isn't sufficently clear on the answer otherwise.