Proper Gauge for bangles

Hey Guys,

What would you suggest as a proper gauge/mm thickness for plain
round bangles?

I think 4mm would be too thick and I think 1mm would be too thin.

Another problem that I have, is that I cannot harden some of my
smaller bracelets [sizes 14 gauge and thinner]. Should I just put
them in the oven at 575 for an hour to harden them?

Please let me know.