I don't think there is anything wrong with your fixer. What you need to do is stop touching the negatives with anything. Never squeegee wet film with one of those film squeegees or your wetted fingers, or photo chamois or sponge, NOTHING. when the film comes out of the can after processing, including wetting agent, shake the crap outta the reel to remove any excess water, then hang to dry in a dust free environment. Use a puffer or canned air to remove any dust prior to enlarging, or anti-static machine. I run an air cleaner constantly in my DR and don't have any problems with dust, hence no scratched film. It's a good idea to check your camera for rough spots and any other gear your film may come in contact with.
If you are concerned with the emulsion being soft, use a tanning developer such as Moersch Tanol, or Pyrocat-HD. They toughen emulsions better than adding a hardener to the fix or stop, and give the added benefit of masking grain.