LucasForums is a bit unique in that all of its communities are quite isolated from each other — most of them 'belong' to completely different sites, and thus different people use each one. Most people find themselves in a certain LucasForums community initially you see, rather than actually stumble upon LucasForums itself.
You should really just look for the kind of people you want to discuss something with, and then find the appropriate forum in that community. So if you want to talk with awesome adventure game-loving people, posting in Mojo's off-topic forum is the way to go.
If you'd prefer to hear from Star Wars lovers, head to one of their off-topic forums.
A moderator shouldn't have a problem if you decide you want to talk about the Wii in two different communities at the same time.