Did some more reading, and I have come to the conclusion that it is impossible for the Corleone Family to be one of the Five. Why? It says that as host, Don Corleone was already there before anyone else began to arrive...and the Five Families arrived last. So it's impossible for the Corleone Family to be one of the Five.

And I also don't believe that the Bocchiccios could be counted among the Five, simply because of the way they act as insurance. When Michael went to speak with Sollozo, a hostage from the Bocchiccios was sent to the Corleones. If Michael was killed, the hostage would be killed too, and the Bocchiccios would blame Sollozo. The same happens here. When we are introduced to the Bocchiccios and this system of negotiations, we learn that each Family represented has been issued a hostage for insurance. So why would the Bocchiccios take one of their own as a hostage? Nothing comes of it except losing a member.