Boo was 5 years old when we got Gracie Ruth. And although they play well together, G really did get on Boo's nerves sometimes with
all the puppy antics.
Yogi came along when G was 9 months old. This has been great for everyone. Boo can play when he wants and G won't continue to pester him when he's ready to stop. Yogi and G play constantly. Their energy levels are the same, their close in size and they like doing the same things. The only problem is that Yogi
really likes to pull Gracie's hair. He'll pull and pull until she starts crying. I'm amazed that she doesn't attack him when he hurts her. But we been very watchful and break it up most times before it gets to the crying stage. Yogi is 4 months old now and the hair pulling doesn't have quiet the fascination it use to. Now it's all about the chase.
I would recommend a boy for two reasons. First, there is rarely a problem with the boy/girl combo. I know a lot depends on the personalities of the Yorkies, but for the most part I think you'll have a better chance of them getting along if they both aren't female. Second, I find that "generally" boys are more cuddly then girls. Many girls are very cuddly, Annie was my velcro dog. Sadie and Gracie love to be loved on, but were/are more independent then either Boo or Yogi. I love my Gracie, she's my special angel, but I love my Momma Boys too!! They are the bestest for kisses, hugs and cuddles.
Good luck and keep us posted with what you decide.