When is the "correct" time to use the less formal Sarge? Chief?

Frankly, I never liked it and never used it. Sounds like you're trying to live the Beetle Bailey cartoon. Sergeant just isn't that hard to say.

You can always call a Navy Chief Petty Officer “Chief”. Now, as for those other ranks that sometimes seem to think they’re Chiefs.... not sure.

Honest, when I'd see senior NCOs, I'd always do full title, however, where I was CMSgts were occasionally referred to as Chief, I did hear that, same with senior warrants once in awhile, though I also heard Mister, however, we didn't have many Army or other warrants where I was, typically...we did have other svc groups on my installations, we had one quite senior USCG O-5 I'd see occasionally, also numerous allied nation commissioned on senior staffs, I can't recall seeing any allied nation enlisted, however...just a few thoughts and observations for whatever their they're worth....