Their relationship in the fanfiction is likely to be full of mutual understanding. Since they are getting married, they must have a deep connection. Takato might be the more gentle and patient one, while Rika could be the strong - willed partner. They balance each other out.
Well, one way this could work in a fanfiction is that Naruto could use Yuta's powers to enhance his combat abilities. Yuta has some really unique and powerful cursed energy techniques. Naruto might be able to combine his chakra with Yuta's powers, like using the Rasengan with Yuta's cursed energy for a super - powered attack. It could make for some really interesting battles against tough opponents like Madara or Kaguya.
It's hard to give an exact time as it depends on the manga's plot development. But typically, Yuta Okkotsu shows up when certain events and character arcs lead to his introduction.