I think Phage should have been aired first after Caretaker - it's a good story that emphasizes the differences between the Alpha and Delta Quadrants, since if they were an Alpha Quadrant Alien of the Week, they could just petition for medical assistance from the Federation and recieve it, while in the Delta Quadrant, the species has to resort to organ piracy. It would have been a good hook, better than a technobabble plot, heavy on the characters or not.
What an interesting idea. Never thought about this before. Perhaps it would have been slightly "bigger deal" if Parallax had come after Phage and B'Elanna had become as Chief Engineer few episode later.