First will Harry ever return to fight Voldemort, or be involved in the ST universe?
Will we see more Wizards or Witches join him?
Yes, he will return to fight, but it's much later in the story. remember he asked the RoR to bring him somewhere he can train and learn to defeat voldemort. It brought him to the ST universe. it's a one way trip so he have to find his way back himself(another reason why i put him in the outer rim of the galaxy near the unknown region.) No, Harry will be the only wizard in the galaxy. remember the setting. the entire wizarding world is in denial including Ron and Hermione. Harry can only rely on himself, his army and the friends he meets in his quest.