Fuming at the new smoking ordinance in Rancho Mirage, restaurant owners in that Riverside County desert community have vowed to fight the compromise law that reserves two-thirds of dining areas for non-smokers. "We need more latitude . . . more flexibility to compete with surrounding communities," said Dominick Zangari, president of the Restaurant Merchants Assn. "It's simply not a fair ratio." The ordinance, which goes into effect June 20, also applies to stores and offices and requires restaurants to install a smoke-filtering system by the end of this year.