You need at least 2 scum because otherwise it is not mafia. You will wamt to have 5 VT because otherwise you will be at evens which is detrimental to town.
6 VT, 1 cop and 2 maf is pretty balanced so I'm not seeing where you are going with adding the miller. You just hurt town's chances to win for no reason at all. If you want to make it balanced I guess you would need to increase the number of VT so cop gets more investigations on average. However, that would not increase the fun factor.
Having fake guilties just isn't that much fun in practice.