When we introduced a newbie kitten we placed the carrier in the room and let the cats sniff around the carrier until the attention died down a bit (not long!) and then let kittie out, he went straight into one of the cat beds in the room (his choice!) and the cats just hissed a bit and ignored him for a couple of days, then amazingly after a couple of days i found them playing together and ripping through the house chasing each other. Always difficult but give it a week and you wouldnt believe how friendly they can be with each other! My youngest cat was 7 months at the time we introduced the new kitten also.