What would be the best type of barrel for my Spyder Sonix paintball gun?

I have a Spyder Sonix paintball gun and was wondering what type of barrel would be best for it. First of all, do all barrels fit all guns?(I mean the part you screw it into.) I dont know much about paintball because I just got into it. My friends keep telling me the longer the barrel the better, but I am not sure. What does a longer barrel do for you and how long should it be for my gun?

What does the low pressure chamber due in a Spyder paintball gun?

I have a Spyder Compact 2000 paintball gun that has a low pressure chamber on it. I was wondering what it did for the paintball gun. If I wanted to add an expansion chamber, would it due the same thing as the low pressure chamber, or would I want to keep both?

All help would be greatly appreciated.