Lucy has arrived! Older cat unimpressed and is staying upstairs in peace.Lucy meanwhile is hiding under the dining table but she pops out from time to time. Some pics are on my profile - hope to get better ones tomorrow when she is more settled
We had a 1 yr old cat before getting another kitten so not such a big age gap. Do things slooowly, isolate them for a few days at first in different rooms, separate litter trays and food bowls etc. They'll be aware that there is another cat about and they'll get used to the scent first. Only let them see each other after a day or so in little supervised 'visits' while they suss each other out and it should be ok.
There may be much growling and yodelling and older cat looking totally affronted at the imposter. It's important that the kitten fits in round the older cat rather than the other way around and therefore acknowledges the hierarchy. Ie older cat is the boss. Make a big fuss of the older cat not the kitten so they know they've not been replaced.
Older cat may biff the younger one about a bit. Ours did a lot of that but no-one was hurt; it was just a way of asserting authority although you'll have to watch and intervene if it's looking a bit rough. I think it can look and sound worse than it is but they do have to establish the pecking order.
After a few days the ground rules have usually been set and you may get the aaahhh moment when older cat grooms the kitten and they cuddle up together.
After losing one of our cats last December we finally went to a refuge today and reserved a kitten 5 months old but looks a bit younger to me. Mostly white but has a black tail. She is coming next Saturday. Any tips on introducing our young 15 yo tortie to her gratefully received!