I'm typing this on my phone so please excuse any typos..
As for the best way to introduce two completely stranger dogs, it is usually done in neutral territory. If you're adopting from a shelter, ask them if they will allow you to do a meeting with Roxy and the potential adoption dog. Some shelters won't (which I don't understand why but oh well) so it's best to ask them.
Or if you get a dog from a reputable breeder or another pet owner who has to give up his/her dog, introduce the two dogs to each other outside, in some neutral territory, like a park. If they both hit it off, that's a good sign. Be sure to get new food bowls for the new dog so Roxy won't have to "give up" her things. Also, bring in new toys. That might help them play together. It really depends on how jealous Roxy can get. Cricket and Spike (my dogs) got along perfectly fine and I didn't do any kind of introduction before I got Spike. 'Course, that was when I was living on my own and Cricket was at my parents' so I didn't have to worry about introducing them. But when I had to move back in with my parents, Cricket and Spike did just fine.
My phone won't let me scroll up so I can't remember if I said everything I wanted to say in the above. If not, I'll see when I get on the computer and write more.
Hope this helps!