How many Screens when using SceneGraph

I have started writing application for Roku devices.
According to forum, docs and blog I should use SceneGraph instead of old API.

After some time I started thinking if I'm doing good putting everything on one Screen.

So there is question to Roku:
Using your new API should everything be done using one screen and components?
Maybe I should use many screens with SceneGraps?
Can I mix old screens API and new SceneGraph?