Google-groups – Backup archive of Google Groups messages

As you've noted, there are three categories that divide these up resulting in 8 different feeds:

15 vs 50 - This just tells you how many items appear in the RSS feed. Depending on how busy the group is and how often your RSS reader checks it, collecting only 15 messages could mean missing out on some if 20 had been posted. On the other hand, if the group gets very large postings a 50 item feed could overwhelm a basic RSS reader.

Messages vs Topics - Like you say, the difference between getting all messages or the first one of each thread.

RSS vs Atom - As a user there isn't really any difference between one or the other. Most of the differences only matter to someone programming the feed creation or reading process.

As a bit of a spoiler, we tested this last night. I have two email address (which I will call for the sake of example "test1@gmail.com" and "test2@gmail.com"), and then we did this:

To:      Rebecca <Recip@gmail.com>
From:    My Name <test1@gmail.com>
Subject: this is a test
Body:  blah blah whatever

and additionally

To:      Rebecca <Recip@gmail.com>
From:    My Name <test1@gmail.com>
Subject: this is a test
Body:  this should nest under the other

and voila, they nested. No work required. And then we did this:

To:      Rebecca <Recip@gmail.com>
From:    My Name <test2@gmail.com>
Subject: this is a test
Body:  does this nest with the others?

And it didn't nest!

So in other words, two emails from the same sender with the same subject will nest. Two emails from two different addresses with the same subject will not nest.