Congrats on adopting a dog!
Somtimes it can be tough to introduce new dogs into your house when you already have pets. It could be due to age, personality, gender, etc.
But in my opinion, all of these things can be addressed over time. Once all of your pets (old and new) realize they are all part of your pack (and you are the leader) they will fall in line.
They may not become best friends, but I'm sure they will learn to live with each other
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