will I be able to tag a post across multiple territories?
Yes. You'll pay posting fees for each territory you post in though - they're additive. If you want to post in ~bitcoin and ~podcasts, and each costs 10 sats, you'll pay 20 sats to crosspost.
if a post was created in one territory but then becomes popular in another territory - how will that work?
If by popular you mean ranked high ...
With personalized ranking, which should get more sophisticated over time, popularity is relative. It can be popular to one viewer and not to another. It can be popular in one territory but not in another.