The latest and greatest Seamonkey (and many other packages for that matter) are located in the testing repo's.
You are correct, enabling all available sources isn't needed. In fact it gets very messy and slow in a hurry. But what that means is there are scores of sources listed that are available to you, but you only need to enable the ones that work best for you. Some work better then others because they may be closer to your location.
You will see that repos are repeated over and over, but with different URL beginnings. The different servers are doing a pretty good job of keeping in sync with the master server, so they offer the same packages.
All of our packages start off thier lives in our "testing" repo. Then once they have a proven record of working properly they are then moved to the main repo's. Generally using packages out of the testing repo is fine, only a small percentage of them ever have any problems. This repo is not enabled by default.
To enable this repo via Glsapt:
With Gslapt running, click "edit" then "preferences". A new window will pop up. Then click the "sources" tab at the top. You will then be presented with a long list of available repo's. The ones with the blue square to the left of them are currently enabled. Now since I don't have a fresh install in front of me, I can't remember if the "testing" repo is there already unmarked. Scroll down and see if you can find it. If you can't find it, you can easily add it yourself.
At the bottom of the same window you will see a text input box labeled "Source =". Copy and paste, or type this URL in the box.ftp://ftp.ousosl.org/pub/vectorlinux/veclinuxe5.9/testing/
Then click the "Add+" button to the right of it. And then click "OK". As the window closes a pop up says something about the sources have changed, and if you would like to update. Agree to it and then Gslapt will read all the information from the repo servers that contains all the information to the packages that are available to you.
Gslapt will return to it's main window with a refreshed list of packages. Then simply search for Seamonkey and all available matches will be shown. Then just click the square to the left of the package you would like to install, and select install or update from the menu that appears. You may then select other interesting packages at this time if you wish. When you are satisfied, click the button that says "execute" and then a new window will show you what packages will be installed. Just agree and your on your way.
It's actually much easier then what I have explained. I'm just hoping to point you in the right direction.
One thing to keep in mind. Whenever you see that we have new packages announced in the forums, you need to click the update button to get your sources current. Gslapt will have no idea they are there until you do so.
Good luck, and enjoy the new packages that you will have available.
The testing link may not be the best for you because of your location. The server is located in Oregon (USA). But it is the master server and will get the job done.
Oh yeah.... I haven't had my fill of coffee yet this morning, and my kids are driving me nuts already. So you can't trust a single thing I've typed.