Cats are territorial pack animals.
Any newcomer to the territory will be viewed with suspision for a short time - hence the hissing and general anti-social behaviour.
With some cats this can lead to a few fights, while dominance is established - obviously not in this case because your kitten is not pushing the issue, and is willing to settle in under the incumbent pack leader.
Once the older cat realises that he is still 'top cat', they will both settle and co-exist quite happily, apart from occasional hissing and spitting which all cats do, and which is part of pack behaviour.
Enjoy your new arrival, and make sur you give your older cat plenty of attention and affection so she doesn't feel pushed out.