The parents of 15-year-old Larry Viscidi, who was charged last month in the slaying of his brother Billy, have hired Arlington attorney Louis Koutoulakos to defend their son.

The entry of Koutoulaskos into the highly publicized Viscidi case probably will postpone a preliminary hearing scheduled for Oct. 10 in Fairfax County Juvenile Court according to Commonwealth's Attorney Robert F. Horan Jr.

Koutoulakos said yesterday he has asked that the hearing be scheduled Oct. 17.

The Arlington attorney, whom Horan described yesterday as a "kingpin's among Virginia defense lawyers, said that Burton and I. Grace Viscidi came to him last week and asked that he assist Vienna attorney Douglas L. Pierson in defending Larry.

After the preliminary hearing, at which the prosecution must show "probable cause" of Larry's involvement in 12-year-old Billy's death, Horan said there could be as much as a two-month delay before he decides whether to ask that Larry be tried publicly as an adult.

It often takes that long.Horan said, for a juvenile court probation officer to make a thorough investigation of a juvenile's social history.

Larry Viscidi was arrested Sept. 13 in connection with the July 25 death of his brother Bill, who died from a blow to the back of the head and was buried in the back yard of his home at 503 Princeton Ter. in Vienna.

Larry was released from police custody on Sept. 18 in the custody of his parents, and now is living at home.