Some doubts

Hello guys, after some reading and watching some things, i have some noob questions.

I can change my stats on barracs but at in lobby leaderboards when im playing with other players, they remain the natural course of improvement. How can i change this? I want to have the stats all the same in the game.

I have seen the great vídeo of distiny, but how can i play with the minigun? do i have to be on a private match or be the host in a game?

thanks for the replies.