You have to edit the default classes in the DSR file directly (Notepad) to make them be used by the game, otherwise it defaults to regular infected loadout. Of course you need to load it as well after that
Change 0 to 1 in gameOpt defaultClasses.axis.[0].class.inUse "0" and infected should use your class (none as weapon is no ammo USP)
gameOpt defaultClasses.axis.[0] is the regular infected and gameOpt defaultClasses.axis.[5] is the initial infected it seems
You can also edit survivor with gameOpt defaultClasses.allies.[0].class.weaponSetups.[0].weapon "iw5_pp90m1" (don't forget to switch inUse to 1) here too
