
Sorry you've had such a rough time of it.
As for introducing another cat to the household, it's not uncommon that the established cat's would resist another cat being brought into the house. Cats are extremely territorial and it will take them a while to adjust to having this new cat around. It's best to leave them separated for now (give the new cat one room of the house) and the other cats their usual run of the house and let them sniff each other out under the door, after a few days you can try and reintroduce them but be prepared for more hissing and growling, it's quite normal and should eventually get better.