A. Any new addition to the household can create a disturbance in the current cat dynamic you have presently. If it is possible having a separate area that can be the new cat's "safe zone" until there is an adequate adjustment period. I agree with expert Julia that products such as pheromone diffusers can help to create a more calming effect. There is also a calming collar product called sentry. If these remedies are not helpful you could consult with an animal behaviorist or veterinarian for further treatment options, that may include medical therapies.
A. It is highly unusual for cats to get on. They are solitary animals and prefer their own company although many will come to tolerate each other some will never get on no matter what you do. You can try by having pheromone diffusers such as feliway around the social areas. Most importantly ensure she has her own room that she can get away from the others with her own feeding bowls and litter tray.
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