Thanks for all your comments, people.
I'll think about it carefully and then get an adult cat. I'm sure it'll be fine!
Swanson will get used to it. He's very stupid, after a month, he'll think the other cat was always there.
Anyway, I asked him last night and he said "marp!" *knead knead knead* "prrrrrmrrp"
*shops for possible cats on adoption sites*