Having read the comments on the health risks, I'll certainly consult Spook's vet before I make a decision.
Thats so not true, until you rescue and rehome adult cats you wont see how well and how quickly they fit into their new homes.
What makes you think I haven't?
My family rehomed two adult cats while I was growing up, and that's where I've formed my opinions. I'm sure some cats are more adaptable than others, but my personal experience makes me wary of adopting an adult cat. The first adult cat we adopted, I believe may have been a former stray. He was very stand-offish, and would bite anyone who touched him. We had him two years until he was hit by a car, and while he became somewhat companionable in that time, and would sit beside you to keep you company, he never liked to be picked up or stroked. Anyone outside the family would be bitten or scratched if they got too close.
The second adoptee was more tragic. Toya was a lovely cat, whose owners has moved overseas and left her behind. She seemed to settle in well, but once we started letting her out, she imediately disappeared. She was found by the new owners of her old house - somehow she'd found her way home 40 miles across a 6-lane motorway. We retrieved her. Keeping her inside full time was not an option, as there were other cats, and young children. The second time she went home, we asked if the new owners could adopt her - they couldn't. The third time, we tried to trace Toya's former owners, but couldn't reach them. After we had had her 7 months, she disappeared (5th time) and never showed up at her old home. We assume she was killed on the road. I think Toya was a one-family cat, and just wanted to find her people again.
Some adult cats are great. I know people who have rehomed adult cats with no problems. Others have significant problems, and while that doesn't mean they don't deserve another chance at a loving home, it does mean that they are not ideal pets for some households. Sadly, it can be difficult to tell if an adult cat will be able to adjust to a new home or not. After our first experience, we looked for an affectionate cat used to children and other pets, and got Toya.
You can't pick a kitten's personality, but you do have the oportunity to socialise it, train it, and bond with it, when it's most receptive. Kitten's aren't going to be everyone's first choice either - some people find them too lively, or want a more independant cat right from the start.
In Spook's case, I think it would be problematic to introduce another adult cat. She's very dominant, and would need to have a laid-back companion that was happy for her to be top cat. A kitten would probrably be adaptable, and used to being bossed by mother cat - an older cat, if it also wanted to be 'boss', would mean constant fights.
Everyone's entitled to their own preference in forming a relationship with a pet - ideally, it's a relationship that will last the cat's whole life. While I'd happily adopt a kitten from a shelter, I've found that it isn't easy - there's a lot of red tape, and I've been told at various times that I need to go on a waiting list, can't have a cat in a rental property, can't adopt from outside my 'district', have to have a home visit, can't have an inside cat, can't adopt 'just one' - must take two, and can't properly care for a pet because I'm disabled. Different shelters have different rules, but all of them seem to have a lot of red tape.
I am interested to get peoples input on whether and how to provide a companion for Spook. Saying that I shouldn't let Spook have kittens because there are somewhere adult cats and kittens needing homes is a bit beside the point, and, I think, a little judgemental. Letting your cat have repeated litters you can't care for or home is irresponsible, making a carefully thought out decision
whether to have one litter of kittens you can care for, and can easily find homes for, in my view, is not.