I don't think you should fall into the trap of allowing them to use a low charisma to have an influence (by being rude or aggressive, especially).
It makes much more sense to treat it as their ability to project their personality. And with everyone at 7, they are going to find it difficult to do so. If they act aggressive people don't take them seriously. Same if they act nice. People don't remember them. People don't ask them about stuff. They've chosen to be wallflowers, and the way that NPCs react to them should reflect that.