My first cat was neurotic, largely because she endured some very rough playfighting when she was a kitten. I learned my lesson, every other kitten was taught swiftly that playfighting was a no-no.
One of the advantages of getting two kittens is that they learn from each other that claws and teeth hurt. They learn the enough-is-enough rule.
If you have room for one cat, you have room for two. Dogs seem to make a room smaller, but cats just use the available space. They sidle, and slide, and drape.
Scratching the furniture: well, I had one chair that was destroyed, but I had it re-upholstered and they never touched it again. No other piece of furniture has ever been used as a scratch post. But then we have doormats (which they seem to love), and trees in the garden.
Boys or girls... well there are advantages and disadvantages to both. Boys spray, but not all boys will spray indoors. Some boys will spray even if you neuter them. Girls can be... well, catty. Girls tend not to get on with other girls. If you decide to get two, watch the litter to see which two get on with each other. Ask the people what they think. If a child is there, they often know more than the adult about the kitts - ask them.
And yes - 6 weeks is too young to remove a kitt from it's mother. When our house was a kitten breeding farm (three litters in two years

), we never let the kitts go before they were 9 weeks old, sometimes later. (This was partially cos we enjoyed them so much, it has to be said). They learn good manners and other important things from their mother and from their litter mates.
Neutering is a good idea, I'd always advise it. Vaccinating is another question. We used to get all out cats vaccinated, and then for some reason, we stopped. It may have been coincidence, but all the vaccinated cats had worse health than those that were never vaccinated. None of our present cats have been vaccinated, and (apart from being neutered) they have never needed to visit the vet.
Litter tray - in our experience, older cats have always needed a litter tray (who wants to go outside to pee when it's cold and wet...?) but the youngsters would rather take a shit far from where they eat and sleep.
Fighting and fussing: well, it's part of the story. Cats are more like housemates than pets. If they get themselves into scrapes and quarrels, there's very little you can do about it. If you have other cats in the neighbourhood, there will definitely be a period of readjustment. At the moment, your yard belongs to some cat. When you get a cat, your yard will belong to your cat. Your cat and the cat who has been usurped will have to work it out between them. If you get a boy cat, this might be more of an issue than if you get a girl cat.
Cats know their own strength and power. It's only when they are more or less equal that they must fight it out. Mostly, they will confront each other, quarrel, complain, sing to each other; full-on fighting is a last-ditch thing.
It's another advantage to having two: two kittens will have more bargaining power than one kitten.
Incidentally there was an
article in New Scientist this week about how feral cats in Rome were observed to give precedent to the kittens when feeding. This goes against the habits of wild cats, who will give precedent to the adult males. The theory is that in the wild, the adult males must be strong and healthy in order for the family group to be protected and safe; while feral kittens must be kept alive and healthy long enough to ensure that they achieve breeding age so that the genes of the older cats are passed on. I thought that was interesting.