I've never had a dog spayed at that age, but I have had one neutered and he did fine. 7 is still pretty young for a small dog, since they can live a longer time so I would say go for it. I understand the apprehension since you already lost a dog, but she was in a compramised state and having problems that would have killed her if you didn't have the surgery. You'll be preventing this one from getting pyometra, uterine cancer, ovarian cancer, mammary cancer, etc. which will put you in the position to either spay her then or euthanize her so it's better to take the chance with having her spayed before she gets these problems then to wait.