jenns:The breeder suggested I should wait a year before neutering Zack because of his small size and risks with the anesthesia. I'm a little hesitant to wait that long though because I am afraid of marking behaviors developing.
As long as you have an experienced surgeon, get pre-op bloodwork, and use both gas anesthesia and a heart monitor, I wouldn't worry too much about neutering at 6 months. Experience allows the vet to operate more quickly. The bloodwork would catch kidney/liver problems. The gas anesthesia allows the dog to be sedated as little as possible.
When you ask about experience, be specific. How many dogs under 5 lbs have you neutered? Complication rate? Fatality rate?
jenns: Also, only one of his testicles is descended. Is it dangerous to wait that long in that case?
Waiting until one year shouldn't be a problem with the testicles. The undescended testicle might still come down.
As long as an undescended testicle is removed by age 2, I wouldn't worry about it. Cancer there can usually be removed cleanly.
However, you are right to be concerned about an undescended testicle. My sister lost a sheltie at 6-years-old due to that problem. The rescue she had gotten the dog (age 2) from had assumed the dog was neutered. Imagine the shock of the vet who did exploratory surgery and discovered the "mass" was the testicles. Unfortunately the dog's tumor had destroyed his immune system and he was unable to recover from the surgery.