Koma are much more satisfactory than fabric or felt tubes as these tend to slip. Man-made threads are quite light in weight. Do not use silk embroidery floss for couching metal threads. Use a fine quality Japanese hand sewing silk. Also it is not necessary to wax the thread for strength.
Silk is one of nature's strongest fibers, stronger that steel, and wax discolors the thread.
Real silver threads will tarnish, real gold ones will not. Use a special pair of scissors to cut metal threads and use them for no other purpose. The metal will chew up scissor blades and make them unusable for any other embroidery. Disposable blade cutters and single edge razor blades can also be used, carefully. A piece of heavy felt on your cutting surface helps when cutting metallics.