With a working strength ph of 5.0 - 5.5 without hardener, Hypam is somewhat acidic. Adding the hardener will make it even more so: not good. It is recommended (but not essential) to use an alkaline fixer for Pyrocat-HD (and other staining developers) as an acid fix is alleged to eat stain. In any event, you don't want to make it more acidic than it already is by adding hardener. In the same vein, I suggest you use a plain water stop, not acid.
Cheers, Bob.
P.S. I should add that this is all "book knowledge" based on what I have read - I've only just started using Pyrocat myself...