Eight Southland doctors were ordered to halt the practice of medicine, and operations at seven medical clinics were cut down or halted pending court hearings on alleged "gross violations" of laws governing prescription drugs. The orders were granted by Los Angeles Superior Court Judge Norman L. Epstein in connection with civil suits filed by the state attorney general last week as a result of a two-year federal-state investigation dubbed "Operation Rx." Four of the physicians affected by Epstein's orders were among seven doctors who pleaded innocent in Los Angeles federal court Monday to separate charges connected to alleged involvement in street-drug dealing rings. The four, all from the Los Angeles area, are Thomas C. Cloud, 43; William Dupuis, 34; John Berrel Barnes Jr., 31, and Stanley Douglas Taylor, 37. A fifth doctor, David N. Michelson, 38, also pleaded innocent to federal charges. He was not restrained from practicing medicine but was ordered to show cause at a June 6 hearing why he should not be.