I agree with the different weights with different shells. One, because with a lighter weight, external factors play a bigger role and cause a bigger spread, which may be ideal when you are facing more than one guy. Two, because with a heavier weight, external factors play a smaller role, meaning you can make further shots; add that with 3 chances in one shot, you should be pretty accurate. Three, because different weights mean different sounds. Heavier bb's make your gun quieter (I promise you. Try this with ANY gun) so if you want to stay concealed with your gun, you are more likely to be unnoticed with heavier weights of bb's, and you also won't hurt people as badly.
Besides that, I would say get A LOT of shells, because those things are VERY easy to carry around since they are so small, and don't jingle around whenever you run around with them like high caps do. I would say, with a shotgun, ammo should never ever be an issue in airsoft. I would also say work on cocking technique so you don't break your gun, which isn't too hard with those dang plastic bodies.
And uh, with a two man team, I would say, make sure that you guys keep an eye out for longer range guys. It's hard to defend yourself with your springers versus AEG's at 150+ feet. Work more on staying somewhat concealed and moving close up to guys, or staying with a group that can cover you for guys that are further away.