What is usually suggested when introducing cats is to keep the new comer in a separate room (if possible) for a few weeks. After the first week switch the beds or blankets of the cats so they will know the smell.
When you start to introduce them only do short sessions and look for any signs of aggression etc... and slowly make the sessions longer.
When you start to introduce them only do short sessions and look for any signs of aggression etc... and slowly make the sessions longer.