You shouldn't change the story characters much, or you risk breaking the story missions. For the voices, many of the story lines are only present for that particular voice.Say i want to change the spawn rates of zombies in the orig campaign game, what should i edit/do?
In Brimstone mod there's sandbox.xml for that but it's for breakdown levels.
Also is it possible to drastically change the main characters (marcus, ed), more than just their looks as in voice too?
The zombie percentages in story mode are set in rtsevents under fsEvent.UpdateZombiePercentages.