That's the great thing about Borderlands 2, infinite respecs (as long as you have cash). I did start with the fire tree myself. Once you lvl up enough to where you can unlock the final skill, just respec. It is somewhat difficult to do mania from the beginning, at least until you get the skill that let's you run around and throw dynamite when you're in fight for your life. Kreig has a steeper learning curve than other characters but he is amazing at higher levels.