Talk about a party getting a bit out of hand. That Hollywood-meets-Washington-street-people party Sunday will not be serving the usual soup-kitchen fare, even though it will be held at hunger activist Mitch Snyder's embattled shelter. No less than the purple trucks of Ridgewell's will be there with tuxedoed waiters, candelabras and tablecloths. The Vista International Hotel is donating two decorated sheet cakes and hors d'oeuvres for the soiree.

Snyder is hosting a party for Martin Sheen and Cicely Tyson, who will be in town for next week's filming of the movie about Snyder and his Community for Creative Non-Violence. The party will be at his not exactly lace-curtain shelter at Second and D streets NW and his 750 homeless residents are invited. Jeff Ellis, president of Ridgewell's, whose purple trucks are usually seen in more prestigious neighborhoods, said they will be serving clam chowder, sandwiches, rolls with a variety of fillings, fresh vegetables, pastries and macaroons. There will be enough soup for 1,000 and enough of the other foods for 500. Hot apple cider will be the beverage.

Snyder's party will undoubtedly turn into a media event with reporters who have never been near a shelter for the homeless or a heating vent finding themselves in unfamiliar territory. Don Johnson for Pepsi

Pepsi-Cola, doing well in the cola wars, has just signed "Miami Vice" actor Don Johnson and former Eagle Glenn Frey to do a commercial in the continuing television advertising series "Pepsi: The Choice of a New Generation." Johnson and Frey will be taping the commercial this weekend in Miami. It will be directed by Ridley Scott, who directed the movies "Alien" and "Blade Runner," and is expected to be quite futuristic. For the third year in a row, the Pepsi commercial will debut during the Grammy Awards telecast Feb. 25.

The Pepsi company, which promises it is not going to come out with a new-tasting Pepsi, has also signed "Back to the Future" star Michael J. Fox and former "Saturday Night Live" star Billy Crystal for "New Generation" commercials. The commercials in this series started in 1984 with Michael Jackson and last year featured Lionel Richie. A Pepsi spokesman said the ads have been responsible for Pepsi's growing market share, which was helped in 1985 by Coke's decision to develop an unwanted new taste. End Notes

Pia Zadora's coming to town. The critics aren't laughing anymore at the tiny, baby-doll wife of wealthy businessman Meshulam Riklis. Her movie work has been disastrous, but she has found a singing voice and selects songs in the Linda Ronstadt style, and the critics have changed their tunes. Her voice on a new album, "Pia and Phil" (with the London Philharmonic), had rave reviews. Now on tour, her reviews continue to be favorable. Zadora will be in town Feb. 2 for a Kennedy Center Concert Hall stop and will bring a large orchestra with her. That's a big hall to fill, so this should be a test of her drawing power . . .

Gramm-Rudman, the legislation that, it is advertised, is going to cure the massive federal deficit, is a mystery to many people. Over the Christmas holidays, Congressional Quarterly advertised a half-day seminar Jan. 23 that would explain the legislation. The seminar organizers asked for a room for 75 at the Hyatt Regency. Nearly 400 have already signed up and more are calling. Peter Harkness, executive editor of CQ, said no issue has ever caused so much interest. Maybe President Reagan might want to attend so he'll understand Gramm-Rudman, but then it might just confuse things . . .

Queen Elizabeth II will be back in the country in May. She plans to spend a four-day vacation at the Kentucky stud farm of William Farish near Lexington from May 22 to 26 . . .

Singer Madonna has asked that she not be included in two lawsuits that charge her husband, actor Sean Penn, with assaulting a photographer and reporter. The reporter, Ian Markham-Smith, has charged that Penn attacked him with a rock when he tried to interview the famous couple . . .

Evelyn Arroyo was running for a seat on the Hoboken, N.J., school board but getting little attention. So she took a new campaigning tack: She ran a newspaper advertisement of herself wearing a clinging silk negligee and, as one might expect, has attracted attention. People now know she is in the race for Tuesday's election. Just think of the political campaign possibilities this opens up. But then, it probably wouldn't have been of much help to Walter Mondale . . .