I've heard that there may be a higher occurence of spay incontinence when the surgery is done that young, but I haven't seen any statistics.
Bone and joint issues would be my concern. Keep her lean as she grows, and don't overfeed. You want her growth to be slow and steady, so that the long bones don't grow faster than they should. Gentle exercise is good, but I'd try to avoid a lot of jumping, going up and down stairs, or hard running until she is mature.