Twyla Tharp has made surprising new plans, joining the American Ballet Theater as artistic associate effective in August. She will also function as a resident choreographer with the company.

Several members of her own dance troupe will join ABT, others will be dismissed, though she terms the move a merger rather than a dissolution of her own Twyla Tharp Dance Company. She will retain the parent non-profit foundation behind her present company so she can work outside Ballet Theater on film and television projects with any dancer she chooses.Tharp began her association with ABT in 1976 when she created "Push Comes to Shove," a ballet with Baryshnikov that became a popular hit. She is a favorite of Baryshnikov's and has choreographed several other works for the company. The move is seen as one that will give Tharp more time for choreographic creativity.

* A NEW OPERA HOUSE will be inaugurated on Oct. 10 in Cairo, where the old house, built for the opening of the Suez Canal in 1869, burned to the ground in 1971. The Japanese have paid for the Cairo Education and Culture Center, as it is formally known. Besides a 1,300-seat opera house, it has two smaller theater, a museum, library, art gallery, conference hall and artistic training rooms.

The Japanese construction company, Kajima Corp., built the seven-story complex in 34 months, at a cost of $30 million. Magda Saleh, a former prima ballerina of Egypt's leading company, has booked an inaugural season heavy on dance, featuring London Festival, Paris Opera and Bolshoi Ballets. But she stresses that the house will be a home primarily for Egyptian artists.

* JEREMY ISAACS, former television executive, will become general director of London's Royal Opera at Covent Garden. He has a five-year plan to breathe new life into British opera and to offset problems backstage, declining public subsidies and the high price and scarcity of great singers. His calls for developing a family of top-flight singers, many of them young Britons; importing top directors to revise dog-eared productions, and sharing productions with other houses. The Royal Opera House will close from 1993 to 1996 for a $184 million redevelopment of the theater and its surroundings. - Compiled from the wire services by Dorothy Stowe