When cats don't get along, we are all unhappy. When we have cats who are aggressive or obnoxious to other cats, we might remember what their previous cat experience might be like.
The most common problem is when we have a cat who is having trouble getting along, and they make the other cats on edge. We have to get the scared cat (because that is what they are) to see that the new cat or cats do not mean them any harm.
Get the scared cat used to the new cats gradually. Talk to the cats when they meet each other, even if it is still on opposite sides of a mostly closed door.
Then we let the cats think of something else. If the problem is still at the lashing tail stage, but no one is growling, we can distract them with a toy or a treat that they have to both eat together or not at all. The key is to shape the behavior we want, in way that will not add stress to the cats.
Disperse cats eager to mix it up with a spritz of plain water. This will impress upon them that it isn't worth it.
Try ways of bringing down the stress level in the house with Feliway aromatherapy, catnip parties, playing soft music and singing along, anything that lets the cats have shared moments of happiness.
That's the only way the cats are going to change their minds about each other. If there are better memories, and rewards for getting along, that will crowd out the bad feelings they have about each other.
We have to show the frightened cat, and the other cats, that there is nothing to be upset about.
To read more about fixing cat conflicts, see more articles.