Ember.js and Ruby on Rails: Strategy

I need to develop an interactive web app with an admin backend. I have thought about using Ember.js for the frontend and Ruby on Rails (with ActiveAdmin) for the backend.

But i have some questions:

1) Should i put the Ember.js app inside the rails project, or having both separate? Is there any performance difference or something i should know about choosing one of the two strategies? I like to have things as clear as i can.

2) Should i use Ember.js and Rails-API instead? I mean... i think i won't use almost anything about the Ruby on Rails project... But i am confused, as i need the Admin Backend...

I have some experience (a little) on Ruby on Rails, but as i am new to Ember.js, i would really appreciate any help you can give me.