If your plastic plug is hitting the screw, I don't think you've drilled your Pocket-Hole correctly... Maybe your screw isn't deep enough? What thickness of wood are you using? Are your drill collar and Kreg Jig settings correct?
The plug itself should not make contact with the screw... the narrow part of the plug is supposed to fit snuggly inside of a properly drilled the Pocket-Hole providing enough friction to keep it in place. Depending on the moisture content of your wood, this could change slightly... a bit of glue could be used as a safeguard.
Can't answer that (KregRep should be able to help). But I have had good luck with the plugs. Sometimes they fit well, but have a slight gap (1/64th) in the middle. Since I use them in pretty hidden places, that hasn't been a problem.