If the adjusters were snug fits on the housing then they are intended to be their own ferrules and you don't need any. I would suggest that when you cut the housings for any point that you then dress the ends flat and square with either a file or a grinder.

If you're using a file you can make up a slick little holder for the housing by drilling a hole in a bit of wood that is a snug fit on the jacket. Clamp the wood in a vise or to the edge of your bench and push the housing up through the hole and while holding it with the ragged end just barely sticking up push the file over the end a few times to flatten and square it up. The wood acts as a guide for the file.

As for where to put the guide I think I would find it awkward to use it while it's near the frame tubes. I think I'm set it up so that they are either up where you had them before or down in free air just in front of the head tube. Both positions would be easier to manipulate while riding by not having the frame tubes in the way.