I think that would be interesting if scumbag role could be selectable like a starting role in custom save setup, he could have a achievement and a solo win screen associated.
The default number of scumbags for "random neutral evil" would be 0% except if the host change this.
The reasons are because some players like hard roles, someone can be creative enough to create a interesting custom setup with a scumbag and is a bit weird that scumbags are the onyl legitimate role that is completly excluded from a starting setup (citizens without vests are powerless and incapable of proving their role but they are possible at start, I think that scumbags should be possible too).