You definitely need to keep a new bird separate for 30-45 days. As far as adding another female, this might not go well. Female parakeets can be very territorial against other females and can be surprisingly vicious. And if you add a male, you have the concern of triggering her hormones and she really doesn’t need to start laying egg at her age. While 7 can be young for a parakeet, due to inbreeding and poor nutrition, the average parakeet life span is 8-10 years. Your girl is on an excellent diet and gets healthy exercise by being allowed to fly. So if she has strong genes, she will hopefully live much longer than a lot of parakeets. I’m not sure if she interacts with people, but she sounds very happy. While they are colony birds, parakeets can be happy with humans as their flock mates. If you do get another parakeet, I would be inclined to let them each have their own cage, and they can interact when they are out of the cage if they choose to. If they eventually seem to want to share one cage, you can try it out as long as you watch them carefully for any signs of fighting.
Thank you for asking Lafeber,