I had 2 older cats (8 and 9 years) when I adopted 2 kittens, 6 and 9 months old. I had the kittens in a room, just like you're doing, for a week, so they could meet each other by smell only. Then I put a "toy" (a sturdy piece of string with a spool at each end) under the door. It extended on both sides of the door, so they could play with each other. I then put the kittens in a wire cage (with a top on it) in the living room and let the older cats smell them. Everything went well, so I turned the kittens loose and all was pretty much OK. A little hissing, but no fighting. You may want to try the toy thing to let the older cat safely "play" with the younger one. Good luck!