The GOG.com release of Creatures Exodus includes all of the major updates/patches for the game. I believe these are the only official add-ons. Everything else needs to be downloaded manually. I made a list of recommendations , and you might also check out the list [url=http://creaturescaves.com/community.php?section=Resources&view=15]here
As for the Gameware Development downloads, the link you listed shows them all. In order to view the descriptions, you used to be able to go to the Creature Labs section of the site. Unfortunately, their site has been in disarray for years, and now it looks like the pages with descriptions are no longer working. You could try the Creatures Wiki
, which might be able to describe some of the downloads. However, I know that some have more cryptic names than others. I'm happy to try to help you out with some specific ones, though!
As for the file name extensions, you might notice ones that end in .agent or .agents. This is most typical of the egg agents, which also come in .exe format. The latter (.exe) is recommended for most systems, while the .agent method is recommended for Linux systems (which I'm not even sure applies here). All the same, if you do see a .agent/.agents download, save this in a folder that should be located somewhere along the lines of Creatures Exodus\Docking Station\My Agents. Just remember to get the remastered patch first, or any add-ons will not install correctly.
Hopefully this is of some help to you! Best of luck!