One thing to note, which I ran into (and consider it still a bit of a bug, though that was debated): if you have rooms or objects in libraries which have a parent in the main game file, Quest will pull those objects into the main game file when you save (even though they're still in the library), which means the game won't load any more until you go in and delete the duplicate definitions.
So if you want to break your game topology up into separate libraries, be sure to set any references to rooms or objects which are in the main game file via scripts.
A library is a separate .aslx file which is included into the main game file via an <include> element. See:
http://quest5.net/wiki/Include_elementThe library itself is wrapped in a library XML element. See:
http://quest5.net/wiki/Library_elementIf you'd like an example, let us know.
Also, as far as testing goes, note that you can start the player off in whatever room you want. So even if your entire game is in one file, you can still start out in a specific part of it. (Or you could make a "teleport" debugging command that takes you directly to a room...)