Accessing static classes - where to put packages?

Your script will communicate with their scripts. Therefore their script will have to be compiled first. Put their scripts in Standard Assets and your script outside of that folder.

http://docs.unity3d.com/Documentation/Manual/ScriptCompileOrderFolders.html

The phases of compilation are as follows:-

Phase 1: Runtime scripts in folders called Standard Assets, Pro Standard Assets and Plugins.

Phase 2: Editor scripts in folders called Standard Assets/Editor, Pro Standard Assets/Editor and Plugins/Editor.

Phase 3: All other scripts that are not inside a folder called Editor.

Phase 4: All remaining scripts (ie, the ones that are inside a folder called Editor).

Additionally, any script inside a folder called WebPlayerTemplates will not be compiled at all.

A common example is where a UnityScript file needs to reference a class defined in a C# file. You can achieve this by placing the C# file inside a Plugins folder and the UnityScript file in a non-special folder. If you don't do this, you will get an error saying the C# class cannot be found.