So, according to my theory, Kashin Koji could have a low profile and his rivalry with Minato Namikaze a secret and watched and observed him but did absolutely nothing. That would easily explain how Kashin Koji knows how to use Rasengan as he would have kept a close eye on Minato Namikaze and then just copied it.
Kashin Koji knows how to summon toads, that he could have learned from Jiraiya as Minato and Kashin Koji would have both shared the same teacher. Also, it is almost certain that he will also know how to use the Sage Mode of Mount Myōboku. Well, if he has learned this much he would have learned the Sage mode as well. Wow, in my opinion, this theory is the most realistic and the most interesting theory about Kashin Koji till now.