We tried adding another little girl for Sophie to play with and it went well the first couple of weeks but then the one we added became very aggressive with Sophie and after a couple months of refereeing she went to a new home with my niece and is happy to be the only dog in their household. I think more times than not it works out. I think different sexes might be easier, just guessing. If possible I would suggest a trial meeting with the two before making it permanent right away. I know there are many YT members with multiple dogs that may give you better advice so I'm bumping this up.
