Try brushing the "old" kit; then use the same brush one the new one.
They need to smell the same before they will get along.

I so have to laugh at the analogy.. good one!

Cat introductions need to be paced to your resident cats acceptance of
change.  Bring the new one home but put in a room by itself.  Watch your
cats to see how they react to this.  If they are interested and not upset,
intros can go faster... if they are a little upset and wary, talk to them in
a calm voice explaining that you are helping out a new friend, and they  you
hope they can all get along - I don't know why explaining it helps - but it
does.  If they have violent reactions it will take much longer.

give your resident cats praise each time you see improvement towards the
interested but not upset stage.  Food is a great incentive.

Once your cat seems ready.. if it was reluctant, swap something out that the
newcomer has slept on.  If still ready you can introduce the cats.  Again,
if violence is an issue, you can put up a baby gate, or harness the resident
cat.  I have found baby gates to be invaluable as it gives you time to stop
the entrance.  You can usually borrow one from someone or get one at a
second hand shop.  Doesn't have to be totally secure.. just sturdy.

If you are still concerned, lock the newcomer back up in the room and praise
your cat for not biting its head off.  if all things look good, let them
free roam.  Be around to supervise.  once your sure then your done.

There are a lot of good websites out there on this.. google cat
introductions.