It is acceptable or recommended to only use 1 keyword per delimiter? Or can we use a phrase?
car, Audi, Ferrari, Lamborgini
Audi Car, Ferrari Car, Lamborgini Car
What is considered best practice?
It doesn't matter since meta keywords have no effect on a page's rankings.
So, use whichever one you think describes something properly. "Ferrari car" doesn't do that as Ferrari is a brand name and can stand alone. "Blue widget" and "green widget" should obviously not be broken down into "widget, blue, green" as that wouldn't make any real sense.