It's not in the initial pack or in High School Sweethearts... but I also don't think there should be any reference to it by then either?
As far as permanently adding scenes... my understanding is that you can use any scenes that you want - there's no harm in including the scenes from latter stories when you go to replay an earlier story.
The exception are the scenes with the numbers "1", "2", and "3" from the expansions, which should only be used when instructed.
That is correct. Any scenes without numbers can be used. There are specific instructions on the baby card. That card is a bit special.