Introducing a new pet to the family

We are going to have to watch my daughter's two cats for a undetermined amount of time starting tomorrow night. They will probably occupy the second floor of our home while the cats we have will be downstairs. How do you go about introducing new cats or dogs into a family without them hurting each other? The four cats we have know each other and all four of them are fairly old (around 9 years old).

Lämmchen said:

Could you maybe introduce them for a few minutes a day and then separate them again until the next day and only do it while the family is in the room? That way a slow introduction could help them see that they are part of the family?

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I'm concerned because I know one of my male cats can be aggressive towards other cats, though he is a sweetie to people. I would think that it would not take very long at all for their to be a serious cat fight

Lämmchen said:

If your family is in the room all together that could help stop fights before they happen? It might come down to where you can't trust that male to be alone with the other cats at all if he wants to be alpha.

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He's outside most of the time, but comes inside to eat. Then if you don't let him back out when he wants to go out he will pick on another cat and try to beat up the other cat. He's very impatient. I wouldn't want to put a cat door in since he kills things often and has often left up parts of animals he has killed. Once we got a squirrels tail. I think he may have eaten the rest of the squirrel.
We are going to put a regular door at the top of the stairs to keep them separated, but they will have to go into the spare room for now until that is installed.

Just a quick update on my situation. Our daughters cats have been at our house for about two weeks now. They have been in our spare room downstairs. A few days ago we decided to open the door to the room and let them wander throughout the house knowing that they would meet up with the other four cats we own. Both of my daughters cats are males. One has hissed quite often at the other cats, but the other one is quiet and doesn't seem to care much. I haven't seen any of our cats hiss except one time when Casper our male cat met Bear who is also male in the hallway. So, overall it has been going fairly well