what if rikudou sennin had every element

[The Alchemist]

Doubtful. The advanced elemental KKGK were created by further understanding the natural elements and fusing them. That sounds like the advancement of science in our world; there was a menial and rudimentary knowledge known by the So6Ps and he simply passed on his knowledge, the rest followed later.

 

[S A G E]

Having a rinnegan means ability to use all elements, so I'm pretty sure he did have at least mastery of the 5 basic elements. Not sure about mastering all the yin and yang since they are unpredictable element.