When I brought SammiCat home, he spent three days in the extra bath before I let him out with Mia. Miss Mia was 3/4 Siberian and 1/4 wolf, so I didn't want to take any chances. They became the best of friends. I'm sure the cats will, too. A few days in isolation won't hurt the kitten. It'll give her time to get used to the smells and sounds of your household. Tonight make sure she has food, a litter box and a soft bed, then close the door.
With new incoming dogs, I feed the two an extremely yummy food side by side - with supervision, of course. For some reason, eating together cements the fact that the newcomer lives here.