A paroled killer pleaded innocent to 26 counts of kidnaping, rape and other felonies involving 15 girls and women, most of whom were attacked in secured apartment buildings in Los Angeles. A preliminary hearing for Ronald Nelson Melvin, 28, an unemployed construction worker, will be set April 21, Municipal Judge Glenette Blackwell said. The charges involve assaults on girls and women 8 to 18 years old that occurred from the beginning of 1985 through March, 1986, in the mid-Wilshire area, a district attorney's spokesman said. Melvin was paroled after serving seven years for manslaughter in the death of a 17-year-old New York City girl 10 years ago, officials said. He is being held in lieu of $500,000 bail.