"Untouchables" takes lots of liberties with reality. There was a real-life Eliot Ness, and he was a "goody two-shoes" whose "Untouchables" crew didn't take bribes. But Ness almost never hurt Capone because he was such a loudmouth and braggart that Capone always knew when and where the Untouchables were going to strike. And Ness and the Untouchables had nothing to do with Capone's conviction on income tax evasion charges--Elmer Irey, an Internal Revenue Service agent, did all the work in convicting Capone. Ness later became Public Safety Director of Cleveland, a position equivalent to Police Commissioner, later ran for mayor of Clevleand, and lost. He was a problem drinker and womanizer, and died young. Frank Nitti was not pushed off a roof by Ness. He and Jake (Greasy Thumb) Guzik ran the Chicago Outfit while Capone was in prison. In 1943, facing another prison term, Nitti killed himself.
The story of the baseball-bat dinner is far more interesting than depicted in the film. Scalise and Anselmi, two of the St. Valentine's Day massacre gunmen, began sucking up to Joe (Hop Toad) Giunta, head of Chicago's Unione Siciliano, which was Chicago's Mafia equivalent (Capone and his outfit were never a Mafia family). Capone was wary of the Unione as a rival outfit, and treated it with kid gloves. Scalise bragged that he, not Capone, was the most powerful man in Chicago--and Capone heard about it. Capone put Scalise and Anselmi to a test: he invited them to dinner with himself and one of his guys, Frankie Rio. Capone and Rio staged an argument at dinner, and Rio slapped Capone and walked out, without Capone retaliating. Thinking that Capone had gone soft, Scalise and Anselmi ran to Giunta the next day and offered to assassinate Capone for him. So, Capone invited Giunta (whose nickname, "Hop Toad," came from his dancing abilities), Scalise and Anselmi to the famous dinner. He baseball-batted the three but they were still alive when he finished. His bodyguards dispatched them with pistols.