These things really depend on the cat. We got REALLY fortunate with our brother & sister litter siblings. They were bathing each other, cuddling, and could be seen without each other from the very start, so I knew they would be the best of friends, and they certainly are! They play together and get along famously.
Their other two litter-mates were really aloof, though. So, it really depends on the kitty.
As far as moulding personality...yes, there are some things you can do that will pretty much ensure different things with your kitty. For one, if they're held at a very young age (from four weeks on, persay), they will be much more likely to like human company later on. If you massage and hold their paws from very young on, chances are you won't have any trouble if you have to do something with their paws later on (like Soft Claws or trimming nails). If they aren't shown the kitchen counters at all during their life, they might never discover the "wonders" found there. And there are more that I can't quite think of right at this moment, but I think you get the picture.
As far as their basic personality, I don't think that can be changed much. You can, however, have a rather social kitten, and something happens that traumatizes it, and it becomes skittish the rest of it's life. When they're kittens you have to be very careful how they are treated, both by yourself and by others, because it does have at least a little bearance on how they act later on.
Anyway, I hope that answers your question!
Oh, that's another thing...if they're exposed to youngsters (kids) at a younger age, they'll most likely be very tolerant of them as adults. For instance, our two kitties are almost two years of age, and Hobbes is my daughter's little play friend. He follows her around the house and meows for her to wake up in the morning. I don't know that he would have been so fond of her if he'd met her later on in life.