"Salut les filles !"
No, because you are talking to identified girls and it would sound weird: like "hello some girls"
I thought that "des" could also be used when there is no article (as in the answer). "Salut les filles" sounds like "Hello the girls" to me.
How can you notice the difference in pronunciation when they say les filles and le fille?
Can this mean an order to someone go say "Salut" to the girls? Translating to something like "greet the girls!"
No. "To greet" is a verb, while "salut" is not a verb. The verb "to greet" in French is "saluer", and an imperative conjugation would be "Saluez les filles!"