I just had my 6.5 month , 3.5 lb Yorkie Max neutered 2 days ago. He came through with flying colors! You'd never even know he had surgery. I decided NOT to go with the first vet I took him to because he didn't seem comfortable with Max's small size. I called and spoke with 5 other places before deciding on one that is very familiar and comfortable with toy breeds plus they operate on birds, ferrets and other small animals.
I'm very happy that I listened to my "gut" about the first vet. Max had one testicle that had not yet descended so an extra incision was done. Luckily it was down about 90% of the way. The Dr. called it a cryptorchid which I had never heard before. He also pulled a couple of baby teeth and microchipped him while he was under. Of course, they did a pre-op blood test and it turned out great or none of this would have been done.
Your baby is so tiny and it's always scary to anesthetize them even when they're bigger. You'll be able to tell if your Vet is comfortable with tiny ones like yours. If not, call around and see who is. If you find the right Dr. and wait until you're more comfortable everything should turn out fine.
P.S. this is my first post so I hope it goes through.