1. Power Rangers In Space continued the saga of Power Rangers up to its starting point. Megaranger had nothing to do with any other sentai (excluding team up movies). 

2.. Space saw the former Turbo Rangers team up with an Alien named Andros to become Space Rangers. Their mission was to save Zordon from Dark Spector. They fought Astronoma. Megaranger was about a teen who beat a video game that was really a test to find Megarangers. He and his frends would become the Megaranger to fight a evil group of aliens led by an evil scientist.

3.  The Silver Range rin SPD was an old friend of Andros. In Megazord and the Silver Ranger was the user of a protype suit that didn;t function correctly. Later this malfuntion was fixed.