Learning how to politely address a dragon,eg prynhawn da, Draig, is what got me into this course.
Helo is probably more common than Shwmae/S'mae. The latter is like a cross between "Hello" and "How are you?", a bit like UK English "Alright?".
I'd say Shwmae/S'mae can show a little more distance from the person you're speaking to than Helo, e.g. because you know someone less well, because you haven't seen someone for a long time etc. Helo is more neutral and can be used in all kinds of situations, distant or near. More familiar addresses would be things like Haia, Hei, Heia, which you can work out from English, and also Ti'n iawn?/Chi'n iawn? "You alright/OK?".
Just tried an alternative spelling of shwmae that I was taught a little while ago: siwmae. It was considered a typo.....
Compared to shwmae, siwmae would actually be a better (more regular) spelling of it. Etymologically, siwmai would be even better. Nevertheless, shwmae is the majority spelling today and so that’s the one Duolingo goes with. You still see the alternatives spellings like yours in less official contexts though.
Yes, I liked the logical siwmae when I saw it. Perhaps we ought to start spreading it? :-)
I’m afraid this has already been tried by other enlightened individuals but alas to no avail. Sadly it looks like badly spelt, etymologically incorrect shwmae is here to stay. Kind of ironic I chose it as my username huh? :D