There's not rules per se. Some folks are more persnickity than others. If in doubt, ping your opponent or post your foibles here and folks can comment that they're cool or not with what you're bringing to the table.
As for missions: It varies. Folks practicing for tourneys tend to stay away from Maelstrom but I've seen a bunch of it played.
I'm up for some 40k 1850 in the morning and afternoon. i need some competitive practice.
"It saddens me greatly that we must take arms against the peoples of the galaxy. By their deaths, they deny themselves the liberation that is only to be found in total surrender to the Greater Good." -Aun'Va