Cats that will not get along.

Question
The first cat  i had was a kitten when i got her, she was the only cat in the household and she remained the only cat in the house untill she was about 2 years old, at 6 months old she had her first litter of kittens but unfortunatly they all died :( but she was not mean or aggresive towards her kittens, i got her spayed when she was 15 months old, she is now a little over 2 years old. I moved from a house where she was able to go outside and play to a townhouse where she has to remaine indoors. Its a 2 story town house with 2 bedrooms so she has alot of room and she was content, when we first moved in maybe about 3 months later a little 12 week old male kitten started running inside , we fed him outside the townhouse and checked on him, well it started getting colder outside so we started keeping him inside and thats when the aggresion started, its been about 5 months now since we first let him stay inside and he has been inside every since, and she still is very aggresive towards him,she even trys to run outside every time we open the front door and she did not do this at all before we let him in the house, he will stand up to her tho and he has gotten to where when she tries to come after him he will fight her off compared to when she first started attacking him he would try to run and hide. He gets plenty to eat and uses the cat box so i know that he is not afraid of her as much anymore. Just recently my aunt dumped her 8 month old female kitten off on us its only been here about 1 week but its hidden most of the time. We hardly ever see the female kitten because the my first cat the 2 year old is so aggresive, it hisses at the female kitten and trys to chase it when it comes out, its really becoming a problem. The female kitten obviously does not feel safe enough to come out and ive tried seperating them and giving lots of love and attention to each cat but nothing helps, since its winter break i have been home alot more than i usually am so i was hoping that that would help but it really has not helped much at all. My 2 year old female cat is also very teritorial of myself. When the other cats try to come and love on me she gets very aggresive towards me and the other cats, were going to try to find the female kitten a good home because we just can not have that many cats inside. We just recenetly rescued another 8 week old kitten yesterday, it was under fed and cold outside so we let it in, its a 7-8 week old male but the other cats frighten it so horribly bad that it will not come out to eat use the litterbox or even get a drink of water, im really worried that it isnt getting the nutrition that it needs. I plan on keeping the male kitten and my 7 month old male and 2 year old female but i just want them to get along. The older male and female tolerate eachother alot better than any of the other new additons.. but i just need some advice.. im kinda new to this, as my other pets in the past have always gotten along.. so please if you have any suggestions i will try it :D

Answer
Heather,

My advice in this situation is to isolate the newcomer in a room to himself. This is partly for health reasons to be sure that he doesn't have any medical issues that are contagious. He should be isolated with food, water, litter, a bed and some toys for 2-3 weeks. It is perfectly natural for resident cats to be upset by this little intruder to their territory. They just need some time to get used to the idea of having a new friend. It is a good idea to switch up the bedding every few days between the new cat and the resident cats giving everyone a chance to get aquainted with each other's unique scents. At the 2-3 week mark and after this new kitten has seen a vet, you can release him with your resident cats. There may be some minor hissing and swatting, but it shouldn't be anything spectacular. Keep a water bottle handy in case things get out of hand and turn nasty. If that happens you may want to isolate the new kitten for a few days more and try again. A nice thing about isolating the new kitten and spending some one on one time with him is that you will get to know how much he normally eats, drinks, relieves himself and plays, which creates a baseline in case he gets sick.  Hopefully I have answered your question adequately. This is a question I get fairly frequently, so if you need more information you should be able to find it in previous questions that I have answered. If you have any further questions or concerns, please do not hesitate to contact me again. I will do the best that I can to give you a prompt and accurate answer.