Well since I have three, I absolutely think Gracie needs a sibling.
But you may want to consider a boy as the chances of there being a conflict is less. There are many that have all girls or all boys and do just fine. And some who have one of each and have problems. But I think as a rule, and we all know there are exceptions to every rule, that boy/girl combo's work best.
As for the Maltese as a second choice, I think Yorkies get along with pretty much all other dogs, so you picking another breed is fine as long as they are also a small breed and you do your research and know about the temperments and any health issues that breed may have.
Good luck in your search and I can't wait to see Gracie's new sibling!!