While I have brought in dogs that neither I or my other dogs have met and it worked out good, they were puppies granted Dazzle was about 8 months but that is still a puppy, Chaos was about 12 weeks so she was still a small puppy but with both I had never met them before I picked them up so my dogs did not meet them until I cam home with them.
When I got Dazzle I still had Dash and Jazz and Jazz was a cattle dog X border collie mix. She was fear aggressive and reactive to other dogs but there were a lot of dogs that she got along with and she enjoyed as long as they played by her rules.
Jazz who was 12 did not like Dazzle when she met him and threatened him big time and at first he was afraid then he realized that being a sight hound he was much faster then her and he would taunt her. She decided it was best to pretend he was not there until Dash died. After Dash died I think she missed playing with Dash and she started to actually play with Dazzle.
At 14 Jazz was still extremely active and at her check up my vet said if he had not known her since she was a puppy he would have guessed her to be about 6 or 7 not 14. That great Cattle dog blood!
When I had added 2 yr old Phoenix to my pack years before I knew Jazz would have issues with her so the trainer at the HS and I worked for weeks getting her to like Phoenix. When they were getting along well at the HS we even did a home visit with Phoenix which was the first time our local HS allowed that but due to Phoenix being special needs ( deaf and vision limited) and being terrified of most people but warmed right up to me they wanted to give her a chance at a home as they had her for about 6 months with little interest in her because she hid from people when they stopped by her run to see her . During the home visit everything went great Jazz and Dash got along great with Phoenix so I adopted her. The first couple of weeks went great then Jazz started going after Phoenix and attacking her . She never hurt her but she did terrify her and I worked on the issue for over 1.5 yrs and every time I thought we had it fixed and things would be great for a few weeks Jazz would end up attacking her again
I ended up rehoming her with my parents where my dogs would still see her but not 24/7 and most of the time it was peaceful when they were together at my parents. My mom died then a few years later my Dad died and Phoenix came back to live with me Jazz and Dazzle ( Dash had died) This time it was peaceful and Jazz accepted her but they were both old ladies.
I bring up the story because even when you have dogs meet and it seems like they get along you can still have issues.
Having said that when I volunteered at Best Friends for a week. in old dog town I met an adorable older male Cattle dog and he came across as quite the gentleman. He stole my heart as he was so calm and so sweet and was living with a group of dogs so got along with all of them. I would have taken him home with me in a heart beat if I did not have Jazz, but I did. I was her person and this old dog was looking for a new person to be his person so I felt there would be issues there. Dash, Phoenix and Dazzle were all ok with letting me be Jazz's person but cattle dogs tend need a person to be theirs and may not like to share . There was a happy ending for this sweet old dog as one day a poster on this forum was talking about her new dog which was an older Male cattle dog from Best Friends, we compared notes and yup it was the old guy I had met. She gave him a wonderful few last years which I was so happy to know as I never forgot him as that look he gave me every time he put his head in my lap that week was like him asking can you please be my person and it is hard to forget a dog like that.