I haven't been around for a long time, but I'm going to chime in on this too. I had my Max for only 2 years (and we got him at 11 weeks) before I 'got' Madison. I had been thinking for some time of getting a female Yorkie (Madison is not female) for awhile, but was on the fence with the "will it be a good thing or a bad thing" for Max question. Madison came to me by being found out in the middle of the CA desert, starving, matted and incredibly sick. I took him only b/c I was SURE that he must have been lost or stolen and got away and was SURE I'd find his owners. HA! I ended up getting him on a decent diet, vaccinated, medicated and eventually neutered and of course, by then, he was MY dog
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Max was NEVER enchanted by Madison and it's been 4 years since then, and still, I wouldn't call them great friends. Max would be quite happy to be the only dog here I'm pretty sure. Madison seems to be pretty happy to have Max around, but Max really isn't too interested in Madison. They do sit together in the front window when I'm gone, watching for me; but other than that, they're not really buddies.
If Madison had not come into the picture being so needy when he did, I probably would not have gotten another dog, but sometimes when I'm gone for several hours, I'm glad that they at least have each other if they get lonely.
I think you've made the right decision for Becca and as much as I love Madison, I think Max would be happier if he weren't around.