Answer: Don’t introduce them at the same time. It is completely inappropriate to leave a kitten in a room with an adult cat. This might result in the cat being frightened, having kittens of her own and possibly killing them (she would be keeping house).
If you want your new kitten to live with older cats, first make sure they like each other. When introducing two cats together, always do so on neutral ground – outside or in another room where there aren’t any outbuildings or fences around that may cause the two cats to fight… such as inside your house! Make sure it is quiet and calm before bringing either one into the other’s domain.. If there are no fences surrounding their respective domains then they can freely roam about inside those areas without fear of attack by each other; but if there are fencing around a particular area then allow both cats access to this space under supervision for a short period only – just long enough for one cat to fully explore it and become familiar with its surroundings – give each cat some time alone with this space before introducing them again.