So, what I found when using a DisplayLink USB monitor is that I could start X on the monitor fine, because DisplayLink has support in the kernel, but I (1) couldn't use my graphics card's acceleration, (2) couldn't easily have the display extend from one regular monitor to the DisplayLink. I was able to eventually get it working, sort of, using screenclone but I couldn't move windows between monitors, get compositing to work, or use xrandr at all. See these questions for some details on setting it up:
It looks like there is a project to get this working better called hybrid-screenclone which will make it easier to do some of the hacks I described.
Ultimately, I would advise against a DisplayLink setup unless you're willing to try some hacks and experimental proding, and still possibly be let down. Though the chances if you getting it working if you don't want to also use a regular monitor are much better.