I’d get a young cat of similar size who’s used to playing with other cats. A 9 month kitten’s liable to be quite rambunctious, and might overwhelm a much younger or older cat.
Having said that: I’ve generally mixed cats of different ages – reduces the chances that, years from now, you lose them all at once. But I wouldn’t get a really old cat, or a really tiny young one, to mix with a 9 month kitten who has nobody else to play with.
Two young cats will almost certainly get along eventually (for that matter, almost as often older cats will also.) Once in a very long time there’s a combination that just won’t work; but as has been said in the thread, give it time. Cats who appear to hate each other on first introduction may become close friends over a period of months.
IME cats of either the same or different genders may become close friends; it seems to depend more on the individual cats. I have known females who didn’t want to hang out with males, but also other female cats who made best friends with more than one, including with unrelated cats.
The spare bedroom for the new one to stay in at first is excellent. If there’s no significant gap under the door, can you temporarily install a screen door? I’m dubious about the baby gate technique since I think it’s very likely that an eight month cat is just going to go over a baby gate, whether by climbing or by leaping. If there’s a gap under the door big enough to stick a paw through, that’ll work on its own.
That article strikes me as a mix of good and very bad advice.
I have introduced a whole lot of cats to each other, and the only part of that I’ve ever done is the separate room at first.
Whatever feeding routine Oliver’s used to, I wouldn’t change that at all. You don’t want the resident cat to think that their routine – especially their food and access to patting – is threatened by the other cat.
Letting them sniff each other under the door is great. If the door’s got a big enough gap, let them stick paws under it – making sure that new cat can choose to stay out of reach. That entire business about details of the exact amount of visual access is human-oriented, not cat-oriented. The cats will know perfectly well that the other cat is there; they can smell and hear each other. Being able to see each other isn’t going to be a shock that needs to be carefully controlled. What needs to be controlled is physical access to each other.
He recommends introducing the cats in a room in which all hiding places are blocked. Do not ever block off all hiding places from a cat who may be upset. They must be able to find someplace to hide, and may go into utter panic if they can’t, and hurt themselves or anyone else within range. What you want to do is block off any hiding places that you can’t yourself access if you really have to, but leave at least one and ideally several available to the cat – and this is true also of the room you’re putting the new cat into. I once had to rapidly grab a small kitten who was about to disappear into the walls or floor via a small space around a heating pipe, and direct her instead under the bed. (I then jammed a lot of steel wool into the space around the pipe to block that off; but as long as she could hide under the bed, she was fine, especially once she realized her brother was also under there. It took them two days to come out while I was in the room.)
If one cat is cornering the other, in hiding places or otherwise; or if a serious fight starts; you may need to break it up. The blanket technique described is a good one; or try a water pistol, or if real trouble a bucket of water (though that may not be great for the room). Don’t try to pick up a fighting or badly frightened cat with bare hands. Heavy gloves and multiple thick layers of clothing, if you absolutely have to.
About the fighting cats article: some good points, but it can take practice with cats to be really good at telling the difference, especially about the vocals. Some cats hiss and growl very easily; some only when seriously upset. I’ve known littermates, close friends, who had a wrestling match every morning for years – and the smaller cat swore at the larger cat all the time. They weren’t fighting, she was just annoyed that he always got her pinned. Her swearing didn’t have the full fury/terror expression of a cat in a serious fight, but an unskilled person might have confused them. If you hear a yowl that sounds like a fire siren – that’s a danger sign. But a hiss or growl might or might not be, depends on the cat.
An unskilled person might also confuse position of ears, and I’ve seen two entirely friendly cats both leaning back on their haunches swatting at each other before one of them decided to dive in for the pounce. I’ve also known combinations of friendly cats in which one always wound up on top, because they were size mis-matched and didn’t have an opposite mis-match in aggression. Notice whether the cat who winds up underneath ever initiates; if so, it’s likely to be OK. If one cat always runs away and hides and seems afraid to come back out, or if one cat not only chases briefly but keeps chasing and chasing and chasing while the other’s clearly trying to get away, that’s a problem; if it keeps happening over an extended period – days or weeks – that’s really a problem.
If they wash each other before or during the fight, that’s definitely a sign of friendship. If there’s a whole lot of fur raised, the bristling cat is either frightened or being overly aggressive.