My Mac is dog agro on lead. He is generally good off lead - particularly with pups and girl dogs. Clover was a pup and a girl dog.
So I thought we had a fighting chance.
Mac stayed with a friend for the week leading up to Clover arriving, so that he didn't associate this house with his territory. (We had just moved in so this was convenient.) We went to the park nearby and let them meet offlead there. Mac was like, "Yeah, whatever, I want to pee on stuff." Clover was like, "It's a border!!" (the only dog breed she had seen and I think the most 'known' thing since coming off a plane.
They've been fine. We didn't leave them unsupervised together for the first week or too, more so for Mac's sanity than anything else. Mac moped for the first week about it and now they play a lot.
They are almost always seperated at least for sleep time. I think it's important for the dogs to understand they are individuals and have time apart from one another.
I made my decison based on how I know Mac is... I thought off lead, at park he doesn't know, being boring and not making a scene, was the best bet. No incidents so far!