Well, I'll consider this an invitation for my opinion. And my opinion is yes. Add an extra goon to the mafias. From what I can tell, one mafia member is working for the townies in each mafia and they both share a mafia member. So, give the mafia's more of a chance. Or if you're looking to speed up the game, add a killer that can kill anyone and not just a vigilante. It's always fun just to be a rogue.
Look at the numbers. You have 30 people. There are currently 9 mafia members. Plus, with the bomber, you have basically two thirds townies and townie roles. So, make it 12/18. One mafia is going to get screwed anyways with the double agent and mafia working for the townies. Plus, the townies will increase if anyone else happens to sign up in time. Or add the killer. Or give the bomber greater power.
But what do I know? I created the politician role and that worked out splendidly.