I did some research on data security that Android has in it's implementation FDE and FBE. I have a clear picture on how both works, but I need something a little different.

What is the best way to enable data encryption whether it is for Marshmallow(FDE) of Nougat,Oreo(FBE) on first boot? 

What I want is that Android does data encryption on it's own, immediately on first boot, and that later user is not prompted with a password. To have something like default password (which I notice that it exists in AOSP)

One more question: from Android's CDD I didn't notice that FBE is mandatory. Does that mean that FDE can also be used on Nougat and Oreo?

Thanks in advance.
I'll update the question with more info if it is not concise enough.