A boss can name anyone he wants, but he won't. The underboss and consigliere will almost always be capos who have their own crews. They will always keep their own rackets but may have others run it for them. Scarfo was a special case since he was very wealthy from running AC and was backed by the Genovese
I agree with that except Scarfo wasn't really that wealthy. After the Bruno hit there were several high ranking guys that got killed also, which moved Scarfo that much closer. Due to Scarfo's close ties with Bobby Manna, the Genovese advised Testa when making him boss to make Scarfo either Underboss or Consigliere. Scarfo would jump over his Captain in this case Freddy Iezzi. Scarfo was surprised Testa made Casella underboss and Scarfo Consigliere. I think Testa was trying to throw a bone and make a peaceful transition. It ended up costing his life. Philly is really an exception to the rule. Scarfo was made Boss and he chose soldier Chuckie Merlino as his UB and Frank Monte as consigliere. Inside the NY Families this is very rare. Only a few occasions that I know where a soldier is made Consigliere.
