Re: Help!!New cat not settling in well
Ditto what everyone has said. It can take a week or more SEPARATED, to integrate. Depends on whats going on. With what you have described that is what I would suggest.
Socialize with New boy, get your cats used to his smell on your clothing, switch their bedding, feed /give treats to the cats on opposite sides of the closed door of the "safe room".
Cats get to know who is family by smell. The carrier trick can work, but dont do it too fast. See how they behave when smelling each other with the door closed. Hissing? GRowling? YOWLING? How bad it is will determine how slow you need to go. If things settle, use a door stop to prop the door open an inch. Let them find the door open on their own.
Bathrooms or a room not used much work best. Dont know how many cats you have but try to make the new cats "safe room" be a room not "owned" or heavily frequented by your other cats... or pick the least used room by the other cats.
When you do try letting newguy out, if you have a lot of pets, try to introduce him to them individually, if your living space permits. When you do try to let the newguy out, KEEP YOUR COOL. If you are stressed they will pick up on it and associate the stress with each other. Before you intervene in a fight (safely! no hands) make sure its a FIGHT, and not play fighting. Playfighting usually is silent, serious fighting is usually not.
See the "cat to cat" and "introducing a cat to new home"
http://www.sfspca.org/resources/library/for-cat-owners
Newguy was in a shelter for a year... so I am assuming this is a cageless shelter?? If so, the cats may have felt like they had to compete with each other for people attention, food etc.. so he may take longer to realize that theres enough to go around now.
Good Luck!!