Criminal charges have been filed against Ronald Nelson Melvin, 28, in connection with 15 sexual assaults of girls and young women in the Wilshire and Rampart areas over the last 18 months. Melvin, an ex-convict, was arrested as he loitered outside an apartment building on South Catalina Street. The Los Angeles County district attorney's office charged Melvin with 26 felony counts--including rape, kidnaping, child molestation and assault with a deadly weapon in connection with the assaults on 15 victims, ranging in age from 8 to 18. Additional charges involving five additional victims are still under investigation, a district attorney's spokesman said. Melvin, a construction worker who has lived at friends' homes in various sections of the city, will be arraigned today in Los Angeles Municipal Court. Prosecutors will ask that bail be set at $500,000, the spokesman said.