John Kobler's bio of Capone (the best IMO) gives a lot of detail about the Outfit's allies and competitors among Chicago's many ethnic racketeers and gangs. Leaves me with two impressions:
--While the Outfit was unquestionably the biggest, richest and most powerful gang in Chicago (and possibly in the US), it never had a monopoly (or even a near-monopoly) on the booze trade;
--Chicago during Prohibition was an OC "equal opportunity employer."