Salvadoran rebel leaders, angered at a remark by El Salvador's president, postponed a meeting scheduled for Sunday with government officials to discuss ways to carry out a new regional peace plan. But rebel leader Jorge Villacorta said the leftist guerrillas will meet informally with government representatives today in Guatemala City. The rebel communique announcing cancellation of Sunday's meeting charged that President Jose Napoleon Duarte had "referred to the Oct. 4 dialogue with a negative attitude that does nothing to contribute to creating a favorable climate . . . to reach a negotiated, political solution."