Gretchen,
It sounds like you have created two separate iMovie projects. To consolidate all your clips in one place you can open both projects and simply copy and paste clips from one project to the other.
In an iMovie project, any clips you import are kept in that project file and are only available to that one particular iMovie project.
I'm assuming you are importing from a camera. Have a look at this tutorial. I think it will be helpful. Be sure to play the little video!
http://www.apple.com/ilife/tutorials/imovie/im2-1.html
And to answer your last question, yes, you'll be perfectly fine editing the project on the external drive.
There are more helpful tutorials here:
http://www.apple.com/ilife/tutorials/imovie/
You'll be an expert in no time!
Matt