My guess is that a vet who you trust, (the key word here-TRUST) would be able to answer your questions. I would think that any surgery would be put off until your little one is healthy. My Yorkie was neutered at 5 mo. and my Pommie was a rescue so they neutered him at 3 mo. before I took him home. I thought 3 mo. was pretty young.
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Joanne

Eli

Chipper

Eva

Snowboy

Rosie(R.I.P. 1996-July 29, 2013)

Cocoa(R.I.P 1998-April 26, 2012):