Can anyone tell me if it's possible to adjust freestyle in the compositor?
I'm using it in my short film and its a drag to actually render the whole thing every time to see the changes I made.
Yes, it's possible. I mostly render my freestyle stuff on a separate Freestyle-Renderlayer and recombine it with the ordinary one in the Compositor. If you just want to rerender your FreeStyle strokes you can just render the Freestyle Renderlayer.
I've only done it in Cycles so far, so the following is only for Cycles.
To separate it:
In the ordinary RenderLayer:
So far for separating. We now have a renderlayer that's transparent and only the Freestyle stroke will be visible.
To recombine it, do it like this in the Compositor:
Note that the Environment Pass is also readded here. This can just be left out if one doesn't want it.
Hope this helps.