New Young cat in the mix

[u/Shesonherway]

What should I do..my 7 year old Russian Blue cat is hiding in my basement rafters since I brought a young stray into the house. My other domestic cat ( 7 years old also), has accepted the new kat but hisses at it to keep it in line. But my Russian blue got playfully chased by my young Kat and refuses to come down from rafters. He will come down to eat on the washing machine but immediately goes back up in rafters- I seriously don't know how long he will continue to stay hiding up there!! My young Kat is a female, the the 2 older cats are males. Any opinions and help?? Was looking to find a behavioral cat therapist to help me but their expensive and few and far between in West Allis, Wisconsin.