Any theories on when they encountered the Kuubi?
Heres mine...
So far this is what we know about the Kuubi's history with the leaf villige.
Up until he was defeated by Hashirama, the 1st Hokage at the Valley of End, Madara possessed the Kuubi.
It also appear he did not have it seal with in his body but merely controlled it with his Sharingun. After Madara, the 1st Hokage Hashirama possessed it (it's still unknown how long he it before he seal it in a host). After Hashirama, his wife Mito Uzumaki became the Kuubi's Jinchuriki host. And after Mito Uzumaki, Kushina Uzumaki (Naruto's mom) became he Kuubi's Jinchuriki host. Tobi stole it from Kushina and sicked it on the Hidden Leaf Villige but the 4th Hokage Minito seal it inside his new born son, Naruto Uzumaki and to date Naruto is the Kuubi's Jinchuriki host.
The only possibilities I see here are that the Kin Gin Brothers fought Madara early on in the Leaf Villiges history (after the Leaf was founded but before his fight with Hashirama.) but that time period is quite brief and its been stated that the Uchiha and the Senju of the Forest form the Leaf Villige first and then everyone else started doing the same and it gave birth to the great nations. So my theory is that the Kin Gin Brothers tried to steal the Kuubi from the 1st Hokage or atleast fought the 1st hokage and thats how they encountered the Kuubi. Hashirama must have summoned the Kuubi and order it to eat the Kin Gin Brothers. The Kuubi was unable to digest them and eventually throw them up (idk how or where). This is also they reason i believe they could not except a treaty with the Hidden Leaf and staged a coup and tried to ambush the 2nd Hokage Tobirama and 2nd Raikage.