I feel your concern, but as a shelter person myself I can tell you that it is frequently (most states) the law that we cannot finalize until the animal is spayed. I agree that it is often too young, but that is the reason my Tar was neutered at 14 weeks, even with me being the executive assistant at my shelter
If it makes you feel better, I have never seen any really ill effects from the animals we have sent to be altered at such an early age. Would they have grown bigger? Maybe..who knows?
Also...if she has an URI (and she most likely does..bordatella is a fact of life with shelter animals) the vet or clinic will probably opt to postpone the surgery. Make sure that they know that she is already pre-adopted. At the clinic we use they will sometimes go ahead with surgery on an unadopted animal depending on how bad the URI is, but they will usually NOT do the surgery if the animal has already been in a home and is pre-adopted. You can guess the reasoning behind that one
Thank you for adopting your baby!!