Introduce them slowly. Initially keep them segregated in different rooms. I recommend putting the new cats in a room and letting the cats that lived there roam and get used to their smell through the door before socializing them further.
I also find meal-time introduction can be helpful. If they are put in the same room with separate food and are very hungry, they may not bother checking each other out and focus on the food instead and get used to each other more slowly.
If they just won't get along, there's not much you can do about it but you can try to at least get them off on the right foot by not triggering more territoriality than you need to. Easing the new animals in will be easier on the existing animals and make them feel less threatened.