What a bunch of questions..wow!
There are, as I understand it, two main schools of thought here. One is the more the merrier, one big discussion group gets everyone involved, or at least there are leaders in discussion and followers. The leaders discuss ideas and the followers read and, sometimes, apply the leaders thoughts to their work.
The other school is breaking people down into similar or mixed types. Put all the potential leaders in one group and range each group down to those who barely speak, based on age, interests socio-economic standing or whatever kind of division you want to apply. Or put one or two or three potential leaders into each groups and randomize the rest.
Depending on your audience, either way can work, just select the better one that suits your situation and audience.
It may be that you could do what is done here, in these forums, a general forum and then more specific forums for specific interests. In an ESL class, it might be the general forum covers everything, with a forum to discuss movies, another to talk about books, another to discuss the daily headlines, another to discuss magazines. However you want to set them up.
There is, I suggest, no correct way of doing things, that will suit everyone, so terms like "best practice" and "world standard" are a nonsense. What works in your classes is best practice for that class, yet that will vary from class to class, cohort to cohort. What works today, won't tomorrow, so what yields the best results? We should never consider that we have a winning formula for achieving the best outcomes for all our classes, for all time. We just do the best we can with what we have and if something doesn't work, change it and move on.
As long as your students have a clear idea of where they are, and where they need to be, a direction and a goal, then most of the time, things will work out.