don't get a kitten! Your older cat will hate you. They are way to high energy and will demand that the older cat play with it.
A good compromise might be an older kitten, say around 9-12 months which will be a little less demanding on the older cat but also will be sufficiently kittenish to keep you on you toes. You might also find that, as the older kitten has been neutered, there is less risk of hormone-related tensions arising as they will as ayoung kitten matures but before it is neutered. You will also have less time in which both cats will need to be kept indoors, as clearly a neutered older kitten would be able to go out as soon as it became settled in the household and responsive (well, as responsive as the ever get) to its name.
Am always happy to encourage cat accumulation, as long as you have the right environment and your budget can cover insurance, vet costs and food/welfare of the wee ones.