I'm guessing that Don Altobello was Boss of the one of the Five Families. But if so, which Family--the Barzini Family, Tattaglia, Cuneo, Strachhi? And why did he seem to be on equal footing with Michael?

If he was a post-Massacre Boss of one of the Five Families, presumably he would've either been a puppet of, or gained power, with Michael's blessing, through Michael. As in, he probably sold out his Boss (one of the original Five Families Bosses) to Michael. Which in an unofficial way would've put Michael above Altobello.

And secondly, how the hell is he Connie's godfather? Connie was born around 1925 or so. If Altobello wasn't a Boss until after 1955, how did Vito even know him, especially so close to have been his only daughter's Godfather? If he wasn't a Boss until 1955, he probably was only a soldier or at most a Capo in the 1920s...While Vito was already a Don of his own family.