Is it ever appropriate to cold-email post-docs that have graduate from your program?
Yes, it's fine. High energy theory is a casual, non-hierarchical, small field of ~2,000 people. If you lived in a town of 2,000 people, you shouldn't be afraid of talking to anyone in it, even the mayor. Here that goes doubly so, as they're only a few years older than you and went to the same school.

As an undergraduate I cold emailed plenty of full professors; some of my friends preferred directly knocking on their doors. The best physicist I know started doing this in high school. I never got any negative feedback, besides occasional admonishment to call them by their first name rather than by their title. In the long run, excessive deference to perceived status will hurt you more than any email ever could.