Linzi, you might not be letting her out yet but there is always the risk she'll escape (and I speak from experience) - take the kitten to the vet for her jabs a.s.a.p., and advice re flea prevention, and get her microchipped at the same time, just in case she gets out (and she'll want to - a year's a long time to keep a kitten indoors, she'll want to be out in the springtime!). Microchipping was the only way Rover was returned to me a couple of years ago.
You may well be able to introduce the cats in due course, when she's bigger - but I suggest the garden (with her in a cage) would be better than in the house Tom considers is his territory.