Our county's Department of Animal Services gets lots and lots of kittens in the spring. Many are old enough for canned or bagged food but not old enough to be neutered, vaccinated, and adopted out. If they are kept in the shelter, they cost a lot of staff time to care for, and they don't get socialized to people and houses.
So they ask people to be foster parents for 4 weeks or so. The kitties come in groups of no fewer than 2, with all the supplies and instructions you might need.
We got two siblings last week, whom I'm calling boy cat and girl cat, 3-1/2 weeks old. We lock them up in a child's bedroom with an open upper-half door when they're napping,which is a lot, or when we're not available. We play with and pet them and let them crawl on us to play-fight each other while we're watching TV.
Old cat sniffed girl cat and gave a perfunctory hiss, but you could tell her heart wasn't in it. Middle cat ignores them or watches from afar. Young cat has done more sniffing around them. Girl cat hisses, arches her back, and backs away. Boy cat approaches to sniff and touch noses, at which point young cat backs off and boy cat follows him.
After the first hour, they've been real good about using the litter box, but the whole house smells like cat shit again. Our other cats go outside. It's a good thing it's spring, and we can open windows.
This is boy cat attacking a human boy toy.