Any smooth, hot press paper with a weight of 100# or higher should work. Wrinkling is a problem as is bulging with all papers and the trick that I have found is to use the paper on a cool surface. Heat accelerates bulging.

The acidic hardener will hurt the developre AFAIK. Kodak would only use it as a separate solution with a wash after the hardener. It can also cause fog in the developer.

Hardening is a crosslinking of the gelatin by aldehyde groups or metals such as chrome alum. You can use 5 ml of chrome alum or glyoxal (either one as a 10% solution in distilled water) in the emulsion BEFORE you coat. This should harden the coating as it dries before the process. At least, that is what I do.