Staples are horrible. Ellie got about 20 of them and two twisted. They had to be yanked out and I heard her scream from the back area of the vet's office.
I hate them and I think the only good reason to use them is to get out of surgery fast (and Ellie had a long surgery and she had to get her teeth cleaned after so it was faster to use them).
We used a onesie for her also. The vet said it would be fine. She fit in the newborn (she wiggled out of the neck of the bigger size) but they weren't long enough so an extension had to be sewn on. Please just make sure they aren't too loose in the back legs or they will find a way to lick the incision.