Have you introduced him yet to your female shih tzu? How dominant do you think his personality is? I don't think you will have any problems if he's a laid back personality. I have a male St/Llhasa mix that was a shelter dog, got him when he was probably around that age, and he got along fine with the cats already in residence. I also foster other small dogs from time to time, and I babysit a lot of my friends dogs when they go on vacation, and he's always gotten along fine with them, whether he "knows" them or not. He is a very outgoing personality and extremely laid back. Only times I ever see him hesitant is around big puppies/younger dogs -- I think he's afraid they might step on him. Once he gets to know those bigger dogs though, he's fine. Good luck!
P.S. I just checked the link, looks like you would do fine with him!
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Advice - Adopting adult male shih tzu with current adult female shih tzu
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