Spaying before first heat prevents mammary gland cancer 100%. Maybe that's what your vet was referring to...
Dog Spay Surgery at ThePetCenter.com .
I can understand why you're so worried, cuz Missy is so small...heard that anestisia is ok for dogs up to 3lbs. Some small yorkie moms here on YT(2~3lbs & under), are waiting for their babies to get bigger to spay them.