You can also say, "Ceau!" I am raised by native Romanians, it just depends where in Romania you come from
Noroc should be accepted - I know it's very informal and between friends.... but it does mean hello
It would just create confusion, especially since its main meaning is "luck". Greetings are taught very early and that's not the moment to teach every alternative possible. Imagine learning about "howdy", "yo", or "sup" in your first English lessons! :P
I would say noroc as Hai noroc, a greeting to a pal or chum. Saying noroc on its own would more commonly be used as a "bless you" for a sneeze.
Buna ziua is good day. While it is a greeting, it's more specific than a general "hello"