In the last 3 years I have gotten 3 new cats. We had an older cat (around 8, and male) when we introduced a new kitten (male). It took around 1 month until they were like brothers. It took around less than a week until the hissing stopped.
Then one year later we got a little girl kitten and both of the other cat’s were not too happy, but the it was mostly the older cat that didn’t like her, no fights only hissing and ignoring her, the younger male cat was obsessed by her in the beginning, constantly stalking her, following her around all the time, everywhere. Hissing and so on, nothing serious, soon he realised that she likes to play like him (the old cat had no interest in this) so they soon became really good friends playing and sleeping together. For the old cat it took a full year until he accepted her and would sleep next to her, but there was no problem between them, he was just ignoring her.
This year we last our oldest cat, which was awful, and the younger male cat was grieving him for a long time after, he has become “older” after this. So we decided to get a new cat, so we got an older cat, a bit more than a year old (also male). The new cat had lived alone in an apartment for most of his life, except for around a month before we got him. Then he had lived were there was wild cats, and those cat’s were really mean to him. So his experience of other cats was very negative. So this time introducing a new cat was really difficult, it took weeks and it was really emotionally draining. I felt several times like this will never work. The new male cat was very afraid and didn’t know how to interact with other cats. So he tried to be bossy and aggressive (even though he really is a easily scared cat) The other cats are pretty easygoing, but they became angry because here was this new cat that tried to boss them around when they were more curious. The new male cat was specially aggressive to our little girl cat (she looks a lot like the wild cats that he had met) I kept them in the 2 separate rooms next to each other so they could still smell each other and interact safely from each other. And I switched the cats between the rooms, so they could smell everything in the other room and so they spent time alone, and with me. And when I put them together I only did it in the beginning (after a week or so) for short moments and I was constantly there, and when I did this I used specially good food (separate bowls for the new and old cats, a bit distanced) so that they would associate the meeting with something positive…still lot’s of hard work , patience needed.
It’s important that you let them work out who is the boss, but you have to break up the fights if they get to hard, you are still the big boss. But it ended well but it really took like 2-3 months I think until I could leave them alone in the same room overnight or if we went somewhere. Now around 7 months later they are friends, specially the new cat and our little girl. They both love playing together.
In my experience when I separate my cats ( when they are sick , after operation…) then after a time they are much more dependent on my attention , the come to me, want my attention more. And when they are together again then they are more drawn to the other cats then to me.
Anyways if you want to bring in a new cat in your household, you have to make sure that you have the time to do it, it takes time and patience. In my experience it’s easier to introduce a kitten than a grown cat, but it’s definitely not impossible to do so. If your life is stressful now, then this is not a good time to do it. I would recommend it that you do it when you have a lot of free time/ vacation time.