It is likely the plastic piece is a 'clip' part of the cable guide. It is possible it does not require a screw at all. If the plastic sticks through enough to interfere with the BB, you can break off or drill through the cable guide to remove the clip and just use a screw. NB, throw away that rusty MF'n screw someone found in grandpa's toolbox. The hole in the BB shell of the frame is very probably M5 threaded and a water bottle bolt will thread in and hold the cable guide tightly.
If the screw is sticking out so much it interferes with a BB, cut it, dear Liza, CUT IT. It's not rocket surgery.