Denny's said it has selected advertising agency Chisholm-Mingo Group. to handle its African American consumer marketing efforts. . . . Republic Industries Inc. said it agreed to buy Ed Mullinax Inc., an owner of five Ford dealerships in Ohio and Florida, for about $100 million in stock after obtaining the necessary approvals from Ford Motor Co. and Ford's Lincoln-Mercury division for the merger. . . . Sierra Health Services Inc. said its chief medical officer, Jerry Reeves, resigned to become chief medical officer for rival HMO operator Humana Inc.