I've heard some talk about a “suggested tournament structure” for certain tournaments, such as GPT's supposedly being cut to top 4, or other similar tournament structure requirements. Is this an actual thing, and if so, where could I find these guidelines/requirements?

Events like GPTs, PTQs, PPTQs etc have specific structures, usually outlined in the event's information document. The generic one for GPTs can be found here: http://wpn.wizards.com/en/resources/rules-documents/244

PPTQs are here: http://magic.wizards.com/en/protour/pptqvan15

One thing not mentioned there: For a limited event in either format that only has eight players show up, do you play single-elimination sealed or just run a draft?

Michael, all of that should be covered in the event-specific documents provided to the Tournament Organizers.

If you are going to Head Judge one of these premier events, I urge you to chat about the event docs with your TO, at least a week ahead. (It's a good idea to chat with that TO ahead of time, anyway!)

And, if that's not your role? Don't worry about it.

d:^D