No it's not.

- There are million different formats.

- guids are not required, they are not monotonically rising integers, and there is no length limits on them (I've seen 50kb guid in the wild)

- date is not required

- you cannot fetch articles "since guid 123". If you go on vacation and return, if the feed had too much traffic they are gone, you'll never see the articles you missed except last 20 or so.

- whether article will be in full or just a teaser is fully in the hands of the server