I've had Bentley (boy) since puppy hood and we had spoiled him ROTTEN! I decided I wanted another one and I, so to speak, rescued my Bitsy (girl), and brought her home when Bentley was less than a year old. Bitsy was around 3 years old. Bentley loved her from first sight! She, on the other hand had to get used to HIM. It took a couple days of adjustment but now they are inseparable! The ONLY problem we have is making sure they each get fair attention. I think it should be against the law to own just one yorkie
They are much better in pairs and most are so adaptable! I think you shouldn't worry at all!