Are all the Hadith works other than the Siha-i Sitta (the so-called six canonical collections) inauthentic regardless of the chain of narrators?
It is not correct to say that only the so-called canonical collections are authentic and the rest of the words are inauthentic. There could be authentic narratives in other books as well. Similarly not all the narratives in the so called six canonical collections or even Bukhari and Muslim are authentic. Every narrative transmitted as khabr-I wahid (individual to individual narrative) is subject to analysis and rejection if it goes against the Qur'an, the absolutely authentic sunnah, common sense and any other definitive evidence. This applies to all the narratives including the ones appearing in the sihah-i sittah.