[@The Fox Without] just as [@Zardoric] stated, sages are very powerful and have great potential. I will list what each sage is capable of as soon as I can to hopefully make things a bit more clear. 

As for your other question, yes, the other races do very much exist and mingle freely with one another. HOWEVER. this does not mean that the Zoras would have figured out how to navigate the underwater passages. All races, not just the Hyrulians, RARELY visit the temple ruins, mainly because it is sacred ground and it is very dangerous. Everyone usually tends to stay at the square where the races sell goods at the market.

[@The One] I want to say no to skull kids and the kokiri as usable races because according to the lore, skull kids are cursed to wonder in the lost woods forever and the kokiri cannot leave the forest or they will immediately perish.