So say i am a mystic / advisor.. Can i later buy warrior aggressor if i had a great back story?

You can buy any spec you want, great story or not, but a great story is always better of course. Just lay out the requisite xp.

I'm going to be a bit of a pedant here for a moment.

You can start the game as a Mystic Advisor, certainly. Then, later, you may buy Aggressor. This will give you the career skill (but no free ranks in said skills) for Aggressor. You will have access to the telent tree for Aggressor.

However, you do not get anything from the Warrior. No extra skills, no access to their inevitable signature abilities.

You may only ever have the one career. In this case, that's Mystic.

Technically correct is the best kind of correct.

Yeah, you can buy any tree you wish with enough points. You might run into the occasional GM who might make you justify it (or, the occasional player - I've had a couple of characters who would have it much easier if I just bought the recruit tree, but it simply didnt fit with the story I was telling with them), but generally anything like that is outside the rules. The RAW says 30 points gets you a second of any tree you desire.