The only requirement to unlock the Rinnegan is to have Hagoromo's chakra. Hagoromo was born with Hagoromo's chakra.
Logic.
I'll use the same argument I used before on this matter. If only Hagoromo's chakra was required, than Naruto would have awakened a Rinnegan along with Sasuke when Hagoromo gave them his chakra.
Hagoromo's chakra being the only requirement is not logical, you need a sharingan to progress it. Madara awakened it that way and Oro was theorizing the fact that sharingan became Rinnegan. Evidence from the manga strongly implies its a requirement, random shinobi cannot awaken it just by receiving Hagoromo's chakra.
The truth is, neither sharingan nor rinnegan are logical eyes for Hagoromo to have originated with. The reason being that Kaguya did not have either of those forms of doujutsu. Hamura having Byakugan makes sense because she had one herself. However, she never had a Rinnegan and only awakened the Sharinnegan after eating the fruit. Now whether you believe the Sharinnegan is an ultimate Rinnegan or ultimate sharingan is another matter of discussion, but either way what Hagoromo ultimately received was a mutated version of this eye power, a downgraded version if you will.
Now, just because I feel Sharingan is a MODERN requirement for awakening Rinnegan does not mean it was a requirement for Hagoromo. Being the first user and the offspring of an alien.... its not impossible to think he awoke a mutated Sharinnegan in the form of a Rinnegan. However, I believe its EQUALLY logical to think he awakened a lesser form of the Sharinnegan in the form of Sharingan. Again, it really comes down to what you think the Sharinnegan is, the ultimate Rinnegan or the ulitimate Sharingan.
With all that said, I firmly believe Sharingan is a MODERN requirement to unlock Rinnegan. I also believe that the chances are equal in regard to Hagoromo starting with Sharingan or starting with Rinnegan, both can stand up to logic because both are far fetched solutions anyway. I mean if Rinnegan did come directly from Sharinnegan, than why did its color change? Neither scenario makes perfect sense.
