Yeah, the nightlies are mature enough to be used daily.
The chosen img is correct and the thread How to install CoreELEC gives more information if you are unsure about the installation process.
How to use the Team CoreELEC ceemmc tool gives information about the CEeMMC installation process.
Good luck! 
PS: VIM3 is one of the easiest devices to recover if anything goes wrong, so don’t be afraid of trying and failing. It’s part of the learning process… 