It depends on the breed. Certain breeds tend to have same-sex aggression. Tosa Inus come to mind. However, Tzus are very gregarious animals and I have never heard of a Tzu attacking a pack member simply due to gender. A general rule is that multiple females are worse than multiple males. I, however, have not found that to be true. In fact, I haven't heard much about same-sex aggression in real life, so I think you're safe to bring in either a female or a male into your home. Just allow them to meet properly, and the rest will fall in place =)

I hope your SA is getting better! One thing to try is NILIF, or Nothing In Life Is Free (just Google it). It will help the SA indirectly because your dog will realize that you are a calm, firm, benevolent leader and nothing bad can happen to her while you are thus. Also, it's good to leave and come nonchalantly and do it frequently. Stay calm, stay consistent, and stay positive. Remove anything that induces stress (I had to close the window curtains) and give her anything that will encourage calmness. Stuffed Kongs are a very good choice! My dogs looooooved Kongs stuffed with yogurt, real meat, peanut butter, honey, bananas, etc. Be creative!