Your Sophie sounds a lot like my Sophie ( a tad spoiled). I posted a thread awhile back you could search titled: caution when adding a second yorkie. Sophie was 8 months old when we added another female, Hazel, who was 7 months. Sadly it didn't work out and we needed to rehome Hazel after another 2 months over her aggression issues. It was very difficult having two females at their ages. At first they played well together but after a couple of weeks Hazel took over everything from the couch to food to toys.
Sophie wasn't happy and so we ended up finding a loving home for Hazel and it was the best solution for everyone.