A plan by the Caritas Christi Health Care network to join forces with a nonreligious health organization that would cover abortions and other family planning services for low-income residents is now drawing fire. Several abortion rights and civil rights groups urged Massachusetts regulators to postpone approving the proposal by Caritas, an affiliate of the Catholic Archdiocese of Boston, to offer insurance in the state's subsidized health program. At the heart of the controversy is a proposed venture by Caritas and the Centene Corp., a St. Louis-based company, that would include the six Caritas hospitals and approximately 33 other hospitals and 66 community health centers.