For the onezie - Mya was just over 7 pounds when she was spayed, and I bought her size 3 months - only because she is VERY long and premie wouldn't have been long enough.
The night of she was very sore.. she curled up with me and just shook like a leaf, to the point that I called the vet as I was so concerned.. By the next day she was ok, still sore but mostly back to normal. I sent Oliver to my friends house for the night so I could solely focus on Mya. Having mommy all to herself helped. When she couldn't be on my lap, she was in her crate, to keep her calm.
Good luck! She'll be fine, sounds like you're well prepared
