Uhh... You form a clan by putting a clan tag to your name.
Why there should be an other way to make one? :X
Or maybe I just didn't get your first question.

You can be unofficial forever if you want to.
Becoming official is just an optional choice. So is a private forum after 6 months.
You could make your clan forum yourself.

E: Okay, Marcoyeh, my turn to post first next time.