We simply rely on the ability of the members of Congress to weigh evidentiary matters as they see fit. We trust their judgment. I don't think it's going to affect the issue of impeachment. The problem is now, the cat's out of the bag. Keep in mind that the report doesn't bind any member of the House or the Senate.
Every member is free to disregard or give it whatever weight they think is appropriate.
Starr's involvement in the Paula Jones civil lawsuit was approved because it was relevant to his investigations into Whitewater and improper use of FBI files. They could, but my guess it that Starr must have decided that there was nothing about these other investigations that produced information relevant for impeachment. Otherwise, he would have included it in his report. I'm a little confused regarding the apparent lack of due process that seems to be oozing from the grand jury proceedings that investigated the president. It seems that the president's guilt is the presumption, even though none of the alleged legal infractions have been proven in a court of law.