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In adopting a new cat to the home of a resident cat the following are the best combos:
2 shelter cats who are known to get on.
Female adult and related female sibling.
Female adult and unrelated female sibling.
Siblings but they may fall out when the become adults.
Two female adults (elderly even better).
Worst: two unneutered males! Bingo, trouble normally.
Amount of space is a factor. The more the better to increase chances of success.