Should I buy a kelly 1 1/2C, kelly screamer, kelly 5B, kelly 5C, or Kelly 7C mouthpiece for marching band?

It depends. What is your goal with these? Is it to play louder? Higher (easier)? Is it a certain sound you are going for? Try them out and see if any of them give you the sound you are trying to accomplish. Then buy the one that does that better than all the others.