If you are introducing a new cat to your household where there are resident animals, it is extraordinarily important that you make the transition as slowly and patiently as possible. Throwing Fluffy out into the middle of the house where there are 5 other cats and 3 dogs is not going to make anyone any friends. A new cat should be put in a room by itself with a litter box and food/water with the DOOR CLOSED for several days. Some cats will want to come out quickly and meet everyone. Others will want to be in there for a couple weeks before venturing out. It can take cats several MONTHS to get used to each other and it is to be expected that there will be a lot of hissing, slapping, and sometimes screaming and chasing as the cats get used to each other.  If you have a situation where there is a great deal of severe fighting going on between your cats to a point where there is blood, fur all over or one of them is hiding continuously, you may have a bad mix of personalities in your household. The only real fix for this problem is to keep each cat in a separate area of the house.