I agree with MrsD by putting the new cat(s) in another room for some time. Make sure all the cats get used to each others' smells via your clothes and hands. We let the new ones have their own room for a week. After the second day we let them out to investigate for several hours at a time, then put them back in the newbie room overnight since we couldn't monitor what happened when we slept. Currently we have six rescue cats and did this with all of them and it worked great. We have one who wishes she were the only cat but she "tolerates" the others. Good luck and God Bless You for being the kind person you are.
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