We just adopted our 12 week little girl Yorkie, Mabel to be a part of our family but also as a playmate to our 16 month old male yorkie, Darwin. We were camping, so they met on neutral territory and we made sure to give Darwin extra cuddling. We've only had Mabel for 8 days now, but things are going great. They play tug-of-war and chase
each other everywhere. So far, so good with a boy-girl combo. Good luck with your choice!!
One more thing, we made sure to get a second Yorkie with an extremely similar personality to Darwin. He is playful but loves to cuddle. He's not hyper at all. Mabel is all of those things and I think this is probably the most important thing to think about instead of boy/girl.