Well, to start with, I would use Kodak Hardening Fixer.

Now, hardeners:

Gelatin melts at about 68 degrees F (20 deg C) (Mees, Haist and Mees and James). Therefore your gelatin will swell and melt at virtually room temperature in water. A hardener will do 2 things. It will raise the melting point so that it will no longer melt and it will stick one layer to another. The former is cohesion and the latter is adhesion.

You need both.

Your hardener must be sufficient for both the layer you coat and anything underneath if there is no hardener in the underlying layers. Also the hardener must diffuse downwards and react. So, you need enough for the emulsion layer and the subbing layer, and you need time for it to move everywhere in the coating.

T&E, Trial and Error is often the only way to figure this out.