You can feasibly obtain every skill in the game (and any skills unlocked by you or any alt characters will be usable by any hero you get, or any PvP-only character you make; so obtaining all the skills one way or another isn't a bad idea). However, note that some skills will be absolutely worthless to your character. Namely, most of the skills in the Primary attribute of your secondary profession, because you can't spec any attribute points into it. So, for example, with an elementalist secondary, you won't get much effect from any of the Energy Storage skills.
By all means, play around and try different things as you learn. If you're in Prophecies, it's a very forgiving area in which to experiment with wacky builds. (And in defense of W/E you can get along with conjures and some of the low-energy defensive skills.) But I think as you progress you'll learn that the energy required to maintain a "fire knight" build just isn't worth it.
And a word of warning: The foes at the end of the campaign are highly resistant to fire damage, so you'll need to learn to swap your builds to suit your battles -- that's a major part of Guild Wars.
Welcome to the game. Good luck.
