Well, the only way to keep from bouncing is to wear those tight bras. So, unless you want to sit out the season, you should wear the bras. I really doubt that would decrease your milk supply. Be sure you wear those tuck-in bra pads, because you may leak milk due to the stimulation of the bouncing.
I breast fed my daughter to 13 months and my son to one year, so I feel like I have a pretty good idea what to expect. Although, I was not running like you will be! No way I could have done it - my breasts are way too large and running was really painful. Being on the team sounds fun though, so don't miss out if you don't have to!
I think the suggestion to get fitted @ Nordstrom is a good idea too. One thing I used to do, was wear an underwire bra underneath and a sports bra on top of that.
Have fun playing, hope you win!