When we were considering getting a second cat, my thoughts always went to Mona and how she'd react to another cat in the apartment. She was on her own for 6 years and I knew she'd be difficult sharing the space with another. I thought she was set in her ways but I also thought she'd really like having a friend around. So, after much deliberation, we adopted a 6 month old kitty.
We, fortunately, had space on our side. When we brought Dorian home, we had all his essentials set up already in the sunroom. Our sunroom has a door with a big pane of glass on it so I thought it would be perfect for him and Mona to look at each other with the safety of still being separated. (Photo to the left.)
At first, Mona didn't even catch on that there was another cat in the apartment. It took a few hours for her to be like, "okay, the door is closed and my humans are going inside it and jingling toys without me." Once she saw him, she stared him down. I thought it would consume her but she did leave her post to go to sleep in her usual spot a room over. While he was in the sunroom, we brought in some of her toys and when he was done playing with them, we brought them back out for her to sniff. Getting used to scents is key. Also letting them use the same litter box helped.
Introducing them face to face wasn't fun. It was October and our sunroom isn't temperature controlled so we had to move Dorian to our spare bedroom. We kept visitations brief but Mona did charge like a bison and push him into a corner hissing and whacking him with her paws. We took about two weeks doing the face to face introductions while being in the room with them. Slowly but surely they were able to tolerate being in the same room without hissing or flying paws.
The thing about Mona is that she still doesn't want Dorian to cuddle with her or play too much. She'll go after him and let him chase her, but she has her limits. They do chill on the same windowsill to look out the window (see below) and sometimes lay right next to each other in the hallway or on the couch. I can totally tell that they enjoy having the other around and they'll always have company in each other. Dorian gives Mona something to do, even if it's just something to look at. I think Dorian benefits from having Mona, too. In a way, he looks at her for behavior cues.
The key to successful cat acclimation is taking it slow. Don't become frustrated if there's hissing and the new cat is hiding from your other cat--separate them and try again the next day. You'll really see improvement in two weeks time, I promise. Soon they'll be buddies and keep each other company while you're at work or away from home. Two cats is the perfect number, I think. There's varying personalities and each offers something different to the household.