My Bailey weighed just over 2.5 lbs when he was neutered at 8 months of age. We tried to get him to put weight on, but he wasn't having it. Because we didn't think he could gain up to the desirable 4 to 5 lbs, we decided not to wait any more and go ahead with the surgery. My vet was confident and I was confident in him. Of course I worried, but that's where a little prayer and faith go a LONG way. My vet used the laser so that he didn't lose as much blood and his pain was minimized. Like you're experiencing, Bailey had learned how to hike and had descended already so we were really just waiting for more weight. He had an incision about an inch long and internal stitches, but was closed with glue on the outside. He stayed overnight at the vet and I picked him up the next morning. I did go by and check on him and was able to see him the afternoon of the surgery, and found him to be in excellent condition. After a few licks on the face and a few kisses on the head, I left him behind. He was to be quiet over the next week or so due to the minimal swelling and internal stitches but other than that, you'd have never known he'd had surgery. He was the same old Bailey the next day.
Ask about the type of anesthesia. Make sure they're using the right stuff for these really little guys. Other than that, I'd say, if you trust your vet, you can't go wrong unless it's something that you can't control anyway.