Slowly introduce the new cat's scent before full interaction. Let them gradually get used to each other.
Gradual Process
Give new cat their own safe room with food, bedding, litter box. Let current cats inspect from outside.
Separate Areas
Swap bedding between new and current cats so they get used to each other's smells.
Swap Scents
Feed cats on opposite sides of a door so they associate each other with positive things.
Meal Together
After a few days, do short, supervised meet-and-greets until cats seem comfortable.
Short Visits
Introduce highest, then lowest ranking cat first. Save most territorial cat for last.
Cat Intros
Have toys, treats ready to divert attention. Separate if signs of aggression appear.
Prevent Fights