How do I change the clip length with the iMovie 2011 trailer presets?
Yes, you can do this.  I'd recommend using the storyboard shot list to get as much of the trailer done as you want following the recommended settings/lengths.

Then, when you are ready to take more control, use the File->Convert to Project menu option.  This will transition the Project settings to the normal iMovie project version and you can change clip durations using the "Clip Adjustments" popup.  You can also drag more event footage into the project/change or add transitions, etc. just as you would a normal project.