Thanks everyone. Often there are several ways of doing things and if you take the less optimal path at the start, it is hard to change. I teach reasonably large numbers of ESL students ( 7 to 8 classes of around 50 students) and I think I will need to keep class forums separate as even one class could be a lot of posting, although I would like them to see forums from other classes. That could be the deciding factor - announcements and assignment dates could also vary by class and I may even find one class has lower streamed students that require a different approach.
The meta course idea sounds interesting and I will be playing with the different ways of doing things. I have six weeks until I need it ready..