Ka, a stop bath is not recommended for Pyrocat-HD for two reasons.
1. The alkali used in Pyrocat-HD is Potassium or Sodium Carbonate. Carbonate reacts with Acetic acid stop baths and can form bubbles of carbon dioxide gas in the film emulsion - thus creating pinholes in the emulsion.
2. A stop bath is unecessary with Pyrocat-HD. The working solution of Pyrocat-HD is slow enough acting that it does not require a stop bath.
A water rinse is usually done before fixing to maximize the working life of the fixer.