I agree.
I am not wrong, Madara was talking to Obito, another Uchiha.
An Uchiha transmigrant can evolve his Sharingan into a Rinnegan with Senju DNA (or directly from the Sage), A Senju transmigrant cannot do the same.
The Uchiha's are not born with Rinnegan, but they are born with powerful eyes, with the potential to unlock Sharingan, and if an Uchiha transmigrant receives the Yang portion of the Sage's chakra, their Sharingan evolves into a Rinnegan.
A Senju does not have and can never have the Yin/Ocular affinity, he is born with the Yang/Body affinity. Same thing vice-versa for the Uchiha, they can receive Yang Chakra but they will never have the affinity, that is why they awaken Rinnegan and not Rikudou Sage Mode.
Only Uchiha have the potential to attain Rinnegan, this is a manga fact.
Correct, only Indra transmigrants.
The Rinnegan is an Yin/Ocular ability, only Indra transmigrants (who are Uchiha) with Sharingan can awaken Rinnegan if they receive Yang Chakra.
An Ashura transmigrant will awaken RSM if he receives Yin Chakra.