Nice idea, just try to flesh out Rachel a bit more/ give a bit of backstory, and get her a bit more involved in the story (an example of this would be the Gumball Guardians asking Finn the math question. where did she go?)

A great writing tool is think how you could explain this to ailiens. Or people who didn't watch the show, same thing. What I'm trying to say is that add a little more description, can only be for the story!

Also, it's fine to completely diviate from the vanilla plot line, cause now that you've added an OC, the world is going to be completely different. A nice example is the Adventure Time fic, Adventure Time - Roxy the Human, by nene9131.

Anywho, PM me if you need any questions answered, I'm happy to help.

is great! :3