To what extent can the hadith of "Hasan li-ghayrihi" be used as a proof, and can it, or the "Hasan" hadith, be used as a proof in matters of 'Aqidah (creed)?

A hadith that is hasan li ghairihi (good due to external support) is accepted and can be used as evidence, just like a sahih (authentic) hadith, in matters of creed (aqaa'id) and rulings (ahkam), even if it is lower in rank. Weak (daif) hadiths cannot be used as evidence in matters of rulings or creed, but it is permissible to narrate them and act upon them in stories, virtues, encouragement, and deterrence.