Mine don't always get along all of the time. There will always be some hissing, growling, or small squabbles from time to time. I have found that introducing kittens is much easier than introducing adults. It must be because they're much smaller and seem like less of a threat.

With young kittens it's taken a few days to about 1 week for my other cats to accept them. With an older cat or kitten it can take a bit longer. My cat Sky has no problems with accepting new cats. He wants to make friends with them right away and the new cats end up hissing at him. My cat Storm is usually very hissy and grumpy, Sunny acts scared or wants nothing to do with them, Pearl is mean and hisses, Starr will also hiss but he accepted his little sister Ziggy right away. He would follow her every where and he was so excited that he squealed with delight. It was like love at first sight for him and he still loves her.

You just never really know what will happen or how long it'll take everyone to adjust to the newbie. You should do it slowly but I've also done it quickly and let nature run it's course. They do learn who's the alpha cat and what they can and can't do with the new cats. You also must give your original cat a lot of attention so they don't feel left out or get very jealous. If you do decide to add another cat to your household I'm sure that many of your questions can be answered here. Good luck.