Most people that wear a paintball jersey (including the pro's) wear either a T-shirt or a tank top and you need to remember, when running around in a field regardless if its a woodball or speedball field, heat exhaustion and dehydration will be yer main enemy.

As fer wearing body armor like a chest protector, it would be useless and it would be considered cheating since there is always a chance when the ball hits whatever part of the body the ball will not break....this problem has occured numerous times in the past wit speedball inwhich there is a rule in all speedball games that the use of body armor is ruled out....the only protection needed is yer face, hands and knees 

It's really entirely up to you what you want to wear underneath your jersey. The more you wear under it, the less it will hurt, and the more likely it will be to bounce. Problem: The more you wear under it, the less used to getting hit you will be, so when you do get shot in a non-padded area, it will hurt more. Also, the other players are going to be very unhappy with you, so you won't be so popular. My advice would be wear maybe one t-shirt or a beater shirt underneath it, the paintballs will sting for maybe two or three hits, and then they stop hurting. Don't worry, I did the same thing. Just take a hit or two, because after that, they stop hurting. It goes from *Whap* "OW! That hurt! I'm out!" to *Whap* " Dang it, I got hit. Out!"

Play on.