Toto's a cutie!
Congrats on getting another dog. Shih Tzus are a great breed! As for your question, from what I've heard male/female combos generally get along better. However, it mainly would have to do with Toto--whether he's dog-aggressive or not; that is, aggressive towards other dogs. Also, it would have a lot to do with the way you introduce Toto and the new puppy. I would suggest introducing them in neutral territory (not your home), that way Toto will be less likely to be territorial. Do so with another person present, such as your spouse, etc. Keep both doggies leashed, let them sniff briefly, and then walk. Walking together allows both dogs to release pent up energy. Allowing them to sniff for too long can build up tension, and a fight may emerge. Further, walking can help their hunting instincts to kick in, preventing tension build-up. This is just a suggestion, as i didn't do this with mine, yet Bella was not dog-aggressive and thus was more willing to accept Rocco. Above all else, give Toto lots of attention and continue to give him treats first, feed him first, greet him first, etc. so that he knows his usual routine and his place in your household has not changed. Good luck!!