connecting the metal quarter panel to plastic bumper could cause you nothing but headaches down the road.
i would assume (no expert here) but since they are different materials they would have different expansion and contraction rates. this could cause your seam to one day break apart for no good reason but a hot/cold day.
door ding guards are a easy fix with right stuff - pop em off, weld holes shut, sand, sand, bondo, sand, prime, sand, paint, sand, clear, sand, polish... and maybe one more sand? but not impossible and has been done before.