That is so, SO true. When I was a kid, we gradually acquired cats ('in the cat turns up and asks if it can move in' kind of way). We started with one (a refugee seeking asylum from her family's recently acquired dog), then got a bonded pair of kittens, and after that they just kept showing up every so often. We generally had 4-6 cats at any one time, and gradual introductions were never even thought of. They'd just show up, move in, and got on with it..
I'll throw my vote in with @Norachan and @betsygee - When I was rescuing the "cats next door" who'd been abandoned, I had 54, spread over three buildings, and even with the daily help of my veterinarian, it was a struggle keeping up with them - but the three "sets" of cats got along perfectly within their respective clowders, and that really was the key to managing them - and to socialising them sufficiently so that nearly all were able to adapt, adjust and overcome (a few stayed on here, and with time and patience, grew to understand, as well). When there's infighting, two cats can be one cat too many (just think of that pair from Kilkenny).
An observation with which I suspect norachan and several others here on TCS will agree: there are times when one particular cat can change the fact that we're one cat beyond absolute desultory chaos. In years gone by, my Japanese Bobtail "throwaway" - Noodles - was the peacemaker and the welcoming committee. Any newcomer immediately fell under his protection, and I never met a cat who wouldn't defer to him. He facilitated the introduction of many new kitties over the years, and his passing made two new introductions rather difficult.
Now, the least likely of all my cats - White Tip, a former feral cat herself, and once the fiercest and least social cat I'd ever known - has become the facilitator, welcoming Black Friday, Clawed-Ya and Mirò as though they were her own Family, and even acting as Mirò's protector initially, in the same fashion in which Noodles once did.
It's all about personality - and whether you have a Diplomat Cat who can ease the tensions while a newcomer learns the ropes, however, at a certain balance point, the dynamic seems to take care of itself. Introducing a new cat into a one-cat household can be incredibly intimidating. Adding a newcomer to a four or five cat clowder is generally a great deal easier, since the group thought process already prevails in most cases. Toss a new cat into a ten cat clowder and the chances are good that the rest with just say, "oh, a new cat," and go on their merry way.
The real answer to the question:
is that there's no such thing. A little time, a little patience and Love is all it takes. Back to Virgil again; "Love conquers all."
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I think Shadow was our most recent Diplomat Cat. He was wonderful with Asha, and she still misses him. It was a huge shock when we brought the boys in last summer, and we had to do full on, carefully segregated introductions. It took a full year for Asha to accept them, and even now she's not particularly keen. She tolerates them, though, and babysits them...which is being in the same room so they can SEE her disapproving of them, and growling to tell them off whenever they start doing anything naughty (e.g. climbing the tv set or pulling things off shelves). They obey her better than they do us! Its taken a while, but she's definitely Boss Cat now