Since the Outfit wasn't a Mafia family, they didn't use Mafia ranks like consigliere or underboss. Nitti started (as did others) as a Capone bodyguard. John Kobler, author of the best Capone bio, had him as "treasurer," though I think he used that term rather loosely. Guzik was always business manager and de facto second in command. But, after Machine Gun Jack McGurn fell out of favor with Capone, Nitti rose. Guzik needed an Italian as a partner to run the Outfit while Capone was in prison, and Nitti was the next guy in line.
Interestingly, Al's brother Ralph never rose far in the hierarchy. After Capone got out of prison (as a near-vegetable due to his tertiary syphillis), Ralph was basically given a stipend from the Outfit to sustain himself and his brother.