All EQ2 XML mods are by deffinition "derivative works" and can not be copyrighted.
Entirely original artwork and 3rd-party utilities such as installers, config tools, updaters, etc associated with a mod could in theory be copyrighted, but to my knowledge nothing uploaded to this site has been.
In short, you may freely repurpose and redistribute anything found on this site.
It is customary and courtious to credit anyone who's work was used or heavily inspired your work.
The
FAQ has some good info, including how to upload mods.
If your set is largely identical to an existing set that allows "addons", its probably best for everyone if you comiple the changed files into an addon package rather than a seperate set. This will save you from needing to keep on top of both SOE changes and changes from the original author. You'll only need to worry about those files in your addon
ZIP is the preffered archive. Some authors compress the entire custom UI directory. Personally I prefer not to include the custom UI directory itself as this can cause problems for the user, particularly if they are modding your mod.
Always include a readme with install instruction if they differ at all from the FAQ. Generally a good idea to include one even if the install is standard.
Hope the above helps. Happy modding