My general opinion is that sub-forums should initially be kept at a minimum, and then new forums can be created as the need arises (i.e. when some kinds of topics takes to much place in other forums).
Under the Scribus heading I think it initially would be sufficient with General and maybe the Installation and setup forums.
But maybe I'm the only one with that opinion?
If the forum becomes really active, I think it will be very useful to have all the categories currently listed. In fact, I would probably split the "Raster and Vector Graphics" in two: "Vector Graphics" and "Images" as I think the kinds of problems and discussions are usually quite different for these two.
Congratulations to the people who put this together. It looks very attractive and so far, it seems easy to use.