As I've mentioned before, Altobello has some aspects of the novel's Bocchicchios: messengers and relative latecomers to the States. It seems they would have been arriving in the States just about when Connie was born, but that era seems pretty elastic. As with Roth, Vito's early gang would have been working allied with various people, and some of those would ended up in the Corleone Family while others drifted off. Altobello may have been someone like that, but Sicilian and Catholic and so a plausible godfather for Connie.
Pete, there's a Youtube analysis that supports your contention that Altobello was part of Vito's early gang. It also maintains that he took over the Tattaglia family at some point. However, that would have made him almost 99 during the film which seems implausible.
