We have 4 moggies- MumCat 11, DadCat 11, 2x CatKids 10.
Although I love them dearly, I think 4 is a lot of hard work- grooming the 2 mid-hairs, daily cleaning the night-time litter tray (outdoor cats but kept in overnight), the fluff in the rugs, the tumbleweeds on the wood flooring. The vet bills for normal boosters, let alone emergencies have been eye-watering, and although we only holiday/ stay elsewhere for 1 week per year, it is a lot to ask of a family member to feed/ keep an eye on 4 ninja escape artists (kept in while we're on holiday as they would merely laugh in the face of anyone else calling them in for the night).
However, we are lucky in that ours mainly get along (MumCat was not impressed for about 6months when she realised CatKids were not flying the nest), and a purring mass of bodies snuggled together (where you can't tell who's head or body belongs to who) is a cute sight, and there's always a cat ready for a cuddle when us or a DC is in need of a de-stress.
It sounds as though 1 of your cats would appreciate a closer companion, although obviously it is a risk they might not get along. Your more grumpy cat might also surprise you by tolerating a kitten, but again possibly not. I think I would be more wary of upsetting the peace if your 2 cats were quite close, but as you appear to have quite a split dynamic anyway I don't think it could upset the balance too much, and early neutering should hopefully prevent or at least deter too much territory marking.
Friends who have introduced a kitten to older cats in the household have remarked that it seems to have given their older cats a new lease of life with the younger cat encouraging them to play more?
Felliway has always worked for us in potentially stressful times (building work/ new babies/ holidays), and ensuring your original 2 cats have some safe places to retire to if they tire of the kitten's antics would be a good idea.
Possibly using a cat carrier to introduce the kitten for the first few weeks- allowing kitten run of a shut off room for the day, then swapping rooms so original cats can smell new scent, then having all cats in open area but with kitten in carrier for short time- slowly, slowly for introducing should be the best way to go. I'd also try to feed the kitten in a separate area for first few weeks, so as not to have original cats annoyed at kitten stealing their food (or sitting in their bowls
).