Of course Sage of six paths has connection with toads.. but personally I dislike idea of Sage of six paths having Toad Sage mode and being taught by Great Elder Toad..., because it could decrease Sage of six paths's actions in my eyes.
Honestly I believe that Sage of six paths had pure/perfect Sage mode reached in human way with method just thought out by him. However While some battle against Juubi or at some point Toad, Snail and Snake helped him with something in exchange Sage of six paths learned them Senjutsu but then each of them, Toad, Snake, Snail adapted own way of acquiring Sage mode, as well it caused that Toad Sage mode, Snake Sage mode and Snail Sage mode(if exists) were imperfect and only obtained some upgrades which are brought by entering Sage mode.
THIS CAUSED THAT PURE/HUMAN SAGE MODE WAS FORGOTEEN.. only The Sage know this secret and he died with it.
This Toad is today known as Great Sage Toad of Myobokuzan with prophecy power, This Snake is today known as Great Sage Snake of Richyudou, and Snail is not discovered yet... so I skip that part.
For example:
Toad Sage mode: Strength, Sensing, high jumps, hard skin, speed etc.
Snake/Dragon Sage mode: Agility, Sensing, durability, speed
They seem to be familliar by The manga shows That Toad Sage mode can't use technique used by Dragon Sage mode aka Mukki Tensei, White Rage sound technique..., Same with DSM can't using Frog kata... etc.
Pure Sage mode of SOSP is actually combination of TOAD, DRAGON AND SNAIL(if exists) SAGE MODE together..., and it could put him on par with tailed beast mode...