I adopted Katsumi, a 2 month old Persian mix, at my local shelter about 1 month ago. When I saw her and pet her at the shelter, she wasn’t overly affectionate, but she was calm and seemed to enjoy the attention. I brought her home and placed her in a room of her own away from the other cats in my household and had her that way for about a week. Once she seemed more comfortable I let her out and about the apartment. My 7 year old cat and her ignored each other. However, my 6 month old cat was very curious about her and started trying to play with her. She started hissing and growling and being aggressive. We have tried to pet her and hold her and she acts the same way. I don’t know what else to do to make her be friendlier and stop acting like she hates us all!
Your kitten is likely just frightened by your other cat and hissing or acting aggressively in response to that fear. Every cat is different, and while the adjustment period was quite short for your new kitten and oldest cat, it could take weeks or months before the new kitten and 6-month-old get along just as well.
You hadn't mentioned the genders of your other cats or if any of them have been neutered, but same gender introductions can sometimes take longer, especially if the cat is unaltered.
There are additional things you can try to help them adjust to one another, such as offering enticing treats or toys to create a more positive experience when they are in one anothers' presence. Feline pheromone diffusers and sprays by Feliway can help to reduce the stress in your household.
Do all that you can to be comforting and reassuring to your new scared kitten. Additionally, don't try to force interactions between the cats. If you give them some more time, and try some additional tools, time will likely bring the peace you're looking for in your cat household!