Those small plastic (and sometimes rubber) donuts are used to protect a bike's frame from getting scratched by bare cables slapping or rubbing against them. They are not really for reducing friction between the cable and the frame. They might protect the paint at the openings of the cable guides, but they do not really help that much to reduce friction or protect the paint from cable abrasion on the rest of the cable guides length. If you want to reduce friction between the cables and the bottom bracket cable guides, you should use a section of plastic tubing that you can pull out from brake cable casings that have them. Cut the plastic tubing just a little longer than the length of the cable guide so it it sticks out a couple of millimeters from the ends of the guides that will keep the cables from contacting the surface of the guide and slide against the much more slippery surface of the plastic tubing.
I recently did that on my last Italian steel bike build and it works pretty good. I was able to get the tubing from cable casing scraps that I had from cutting my brake cable casings to length for my brake system installation.