Federico Fellini can come up with some unusual ideas. He is planning a new film "Ginger and Fred," based on the careers of Ginger Rogers and Fred Astaire and starring his wife, Giulietta Masina, and Marcello Mastroianni. He will have 30 doubles for famous people, including James Dean, Queen Elizabeth II, Pope John Paul II, Marlon Brando, Robert De Niro, President Reagan and Liza Minnelli . . .

They love actor Telly Savalas at the Vista Hotel. When they found he was checking in for the inauguration, they remembered that "Kojak" loved lollipops, so they furnished his room with Tootsie Pops. And yesterday, he returned to his room to find a surprise from the hotel staff -- a double chocolate cake decorated to look like a suit jacket opened to reveal a holster and gun, and more Tootsie Pops . . .

New York lawyer Roy Cohn has been making the social circuit in town this weekend. He hosted a party at Dominique's restaurant Sunday night with a guest list that included Sen. Chic Hecht; United States Information Agency director Charles Z. Wick; U.N. Ambassador Jeane Kirkpatrick; and beauty products empress Estee Lauder . . .

Overheard at rehearsals for last Saturday's inaugural gala: Frank Sinatra, after watching ballet great Mikhail Baryshnikov perform, saying, "Boy, he'd be a great opening act for my show" . . .