Well i'd like to deposit what i know for scrims:
traditionally a scrim is 2, 25 min rounds, usually on a small map: ie: refinery, plantation, desolation. All guns are allowed, but tanks are not. Some weapons such as c4, claymores, and nano's are frowned upon, but not against the rules. basically your just trying to get enuf energy then nuke the enemy base. first round one team plays on us team, and the second round they play on nk team. If the teams are super unbalanced, then the second round people may jumble the teams a little for more balance. Those are all of the rules i can think of off the top of my head
Oh yeah, and traditionally the server is clean of any mods whatsoever. (except for ex mod)