Bringing a new cat into a single or multi-cat household can have varying results. Your old cats may simply just avoid the new cat. Or there may be some direct interaction ranging from hissing and arching to all-out attack. These reactions may not be a result of hate or fear.
Domestic cats who are now feral (live in the wild), tend to band together in a large group. The cats within this group do help each other. If a large unknown male cat tries to get close to the group it will be attacked by group members to protect their kittens. Outside males are known to kill kittens from another sire in preference of starting their own family. A cat wishing to join this social group will have to spend months living quietly around the edge of group. Only when the group adults feel there is now danger will they allow this new cat into their group.
A new cat being introduced into the home may garner the same reaction from your cats. They are wary and concerned the new cat may try to harm their group members. This hissing or attacking from your cat is simply a way to tell the new cat they will not hurt the ones they love.
The best way to introduce new cats into a household is to keep them separate at first. Set up a room for the new cat with a litter box, food and water bowl, and scratching post. Allow your existing cat to play in the room so their scent permeates the area. Bring your cat home in a carrier and set the carrier down in the new room. Allow your existing cat to approach the carrier and interact with the new cat. Once your existing cat leaves the room, shut the door and open the carrier so the new cat can explore. Do not force the new cat to exit the carrier. Let them do that on their own. It might be best for you to leave the room too. Give them time to become used to each other’s scent. The crack between the floor and the bottom of the door allows the cats to approach and sniff each other and get to know the other. When the cats seem settled try and open the door. Have a squirt bottle ready with water just in case of conflict but only use it if absolutely necessary.
Unfortunately there may be situations where your cat or the new cat do not seem to be getting along. It may be due to the fact that your cat lives in a single cat home and has simply not seen another cat in ages. Or you got your kitten when they were younger than 16 weeks old and they were never taught proper cat etiquette by their mother. If one of these issues is true it can take months before the cats will accept each other. You will have to be very patient.
The worst case scenario is the new cat will never get along with the rest of the household. It may simply be a matter of personality and the just do not like each other. If this is the case then the best you can hope for is tolerance between then without aggression. It will be important for you to spend quality time with each cat to prevent jealousy issues. If the cats cannot be near each other without fighting ask your veterinarian for a good behaviorist to help solve the situation.