1. I have no plans to have the Serenity MC and Last Human MC meet. There will be other minor 'crossover' situations though.
2. For now, only Rin comes to visit Pantheria. I have ideas for 1 or 2 others to MAYBE visit at some point. But that isn't definite.
3. Not necessarily, no. Those are more like 'fantasy' characters to me, and Last Human is 'science fiction'. Besides, the Captain will have his hands full with quite a lot of other distractions.