In Dynamics of Dispute, Rabbi Lampel points out that in areas of Aggada, we even find Amoraim breaking the Golden Rule and arguing with Tannaim - see for example Megillah 7a in which Shmuel claims he has a better source for the Divine nature of Megillas Ester:
אמר שמואל אי הואי התם הוה אמינא מלתא דעדיפא מכולהו שנאמר קימו וקבלו קימו למעלה מה שקיבלו למטה אמר רבא לכולהו אית להו פירכא לבר מדשמואל דלית ליה פירכא
Said Shmuel: Had I been there, I would have said something better than everything they [the Tannaim] said, as it says etc... Said Rava: All of them [the answers of the Tannaim] have a weakness, but Shmuel's answer does not have a weakness
Rabbi Lampel has a thesis which he uses this to support, namely that the reason in general that Amoraim do not argue with Tannaim is because there was a chain of tradition regarding which halachos were Halacha L'Moshe MiSinai (he proves this with other methods as well), and that chain of tradition was lost in the mayhem of the generation of Rabbi Yehuda HaNasi. Therefore the Amoraim would not argue for fear of inadvertently arguing with a Halacha L'Moshe MiSinai. However, in Aggada there is no such concern, and therefore they felt free to argue. (He also concludes from this thesis that if a given Halacha is clearly not from Sinai, for example if a Tanna had an opinion but then changed his mind about it, then an Amora was free to argue on that Halacha as well, and he cites examples of such.)
This would justify arguing with essentially any authority in an Aggadic discussion.