If a girl says to a guy 'Hallo du Lieber', is it always romantic/flirtatious? Or can it be friendly like 'hey darl' in english? Have heard this and seen it written, and I'm unsure of the context. Don't want to use it myself incorrectly!
Difficult to answer without any more context, e.g. social setting. There is a slight difference to "Hallo mein lieber" as well. I'd be very careful to use it, especially as a male. In fact, I would avoid using it. It could also be perceived as sarcastic, as in "Hallo meine Holde". It might be flirtatious, but in that case I think it is more like English shop ladies using "honey", "love" etc. when dealing with you.
Today, I guess nobody would use "Hallo mein Lieber" to initiate a serious flirtatious with somebody (using "du" in this sentence sounds wrong to my ears). This sentence has (if said to an adult) an ironic undertone, in which case you could say that someone initiated flirtatious while joking about it if this makes any sense.
Same goes for "Hallo mein Süßer / meine Süße".
"Hallo du lieber" sounds like the noun is missing.
I would use "Hallo mein Lieber"/"Hallo meine Liebe", but that also depends on the voice and tone and – like 0x6d64 already stated – can has an ironic/sarcastic meaning.
"Hallo du lieber Bengel." again is used by my grand-mother, to welcome my little son (having a meaning like "Hello, you lovely rascal.") That's the usage I've thought of first, when reading your question.