I would let the new cat get used to the room he is in for about a week or more before you introduce him to your other cats. He may act out of fear because #1 he's in a strange place and #2 he may feel the need to defend his territory from your other cats. Then gradually let them sniff at eachother through the door etc. It just takes time. I've had some cats adjust immediately and some take a few weeks or a month to feel comfortable.

The funniest thing that happened was when I introduced a stray 5 week old kitten to my 4 inside-only cats. She was tiny and non threatening but very curious and my older cats were terrified of her even though she was only half their size. She adopted my only male (16 lbs) as her "mommy" and followed him around everywhere. He hated her at first but never once swatted or growled at her. Just have patience and they should all get along.

Update....

He has been on the sun porch since Wednesday. We had him neutered on Wednesday and got his vaccines, etc. We took him to the vet yesterday to take care of the ear mites and fleas.

We are disturbed about his bathroom habits, though. There is a litterbox on the sunporch that he has been reliably using. However, when we've taken him in the car to the vet, he pees and poops on himself out of fear.

Well, today, we let him out of the sunporch to explore since he is all treated and flea/ear mite free now with the current cats locked up. He had been walking around for about 30 minutes until he sat in an old suitcase and peed. He didn't spray, he just straight up peed. He laid down and just started peeing, and when I picked him up he kept peeing on the floor in a puddle.

We are not sure what to do. He obviously knows how to use a litter box and has reliably used it on the sunporch, but now he peed on the suit case downstairs? I don't get it.

Maybe he went too far and didn't know how to get to his litterbox...the litterbox is on the sunporch upstairs and he was downstairs. I don't know. What do y'all think?

Another update. For the past 4 days we have had 3 baby gates in the doorway between New Kitty's bedroom and the other cat's room/hallway. They can generally sit fairly close to eachother without hissing growling, yowling, etc. But New Kitty escape a couple times and one of our cats just stares at him for a while and won't let him past certain points (which eventually has led to a fight twice) and the other kitty is just particular and yowls but doesn't do anything but hide.

Will they ever get along? I'm figuring since they aren't going after eachother through the gate that means they'll eventually at least tolerate eachother. I'm scared!

I think it is a great sign. If they really wanted to get at eachother, they could/would certainly hop over those gates.
Stay patient- I am sure it will all work out!

My friend brought a male rescue kitten into her house and her other 3 yr old male despised him. he would growl and hiss and swat at him. My friend cried thinking she did the wrong thing and felt bad for her older cat. It took about a month for the two cats to be in the same room together without any problems.

Now, when I visit her, I see the cats rolling around and playing together. I think it was a good 3 months before that happened though.

Thank you for the responses!

There was a new development last night. I came home and was relaxing in the family room. When I went upstairs, Duke (new cat) had somewhow escaped and Baloney (current cat) was just sitting there watching him. Well later that night my partner noticed Duke had a little blood on his ear, must have been a fight with Baloney.

Well, we were feeding Duke and playing with him, and Baloney came into Duke's room. Duke immediately became upset. I clapped my hands to make them stop staring and they started fighting for a good 20 seconds, totally locked onto eachother, hair everywhere. They then ran downstairs, Baloney hid under a chair, and our other resident cat ran away into the family room long before that. I wrapped Duke (new cat) in a blanket and took him upstairs and we are starting from square one now. Duke is in his room with the door shut now, and will stay that way until next Monday. Then we'll start with the gates again.

I am saddened by this. I fear that the cats may never get along and that they are vying for dominance and neither wants to give up. I hope I don't have to give Duke up, my partner loves him so. We are maintaining hope and we realize we are only two weeks into this thing, but it's not good.

There really wasn't any major damage from the fight other than Duke apparently pulled a big chunk of hair out of Baloney and the skin was very raw where that happened.

Stanly