My vet had said 6 months and at least 5 pounds before spaying. Since she is already over 5 lbs and the appointment isn't until mid September I don't see why this would be an issue. It's between yourself and your vet regarding her spay.
Do you have some type of insurance that is covering the cost of her spay? Is this why you are saying she needs it done before 8 months old? Keep us posted how she does