It's called common sense. Hagoromo and Hamura are nothing more than diluted versions of Kaguya,
Seeing as the Rinnegan is a different doujutsu from the nine tomoe, there's no basis for your statement. It's heavily implied the Rinnegan was created via the combination of the nine tomoe eye and byakugan since it possesses aspects of both. The databook mentions nothing of Kaguya possessing the six paths. Nor does she use any six path techniques in the manga. The creation of all things technique is Hagoromo's technique, as was the nine gudou damas. Hagoromo created life with his power, while Kaguya created her own dimensions. It's not the same thing. And the realms she created reference those of the world tree from Norse mythology, not the samsara. Finally, the databokk doesn't even give the nine tomoe a name. The translation leaves "Rinnesharingan" in parentheses because it references the fact that it's an urban suffix created by the fandom, no different from Juubito lols.
being singularly weaker than her in any of their forms.
That was only shown when she was in the form of the juubi, which was when they battled her.
If they had Six Paths Chakra which granted said Gudoudama, then she does as well.
The techniques are not in her databook and she did not use them even when she was cornered. You have no basis. When some elements are combined, they create something new. This was the case throughout the series as well.
Not to mention it was all but confirmed when she took out her massive Gudoudama which was on a scale no other Juubi related character has ever done.
Hers and Hagoromo's gudou damas are not the same thing. His encompass all the elements and creation of all things, which creates life. Hers constructs new dimensions.