In regard to
Yorkies and cats, there is no difference in how a male or female Yorkie will react. Surprising to many, is that this breed generally gets along very well with cats and some Yorkies are best friends with felines housemates. This should, however, always be tested when thinking about introducing a new pet into the house.
In regard to having a male or a female and wondering which gender would be best when adding a second dog, there are pros and cons to both same and opposite gender. Whenever you have two dogs of the same sex, this may create a competitive environment. This is particularly true if the dogs have not been fixed (
spayed and neutered). With two girls or two boys, there can be jealousy. This is less likely to happen if the Yorkies grow up together. This isn't to say that this will happen or that it won't happen with opposite gender; however this is something to consider and keep in mind as a possibility.
On the flip side of the coin, if you have a male or female Yorkshire Terrier and are thinking about getting another one, you will already have experience with the gender of your established dog... and this can make care of the second pet much easier.