...although, as to which is which, your guess is as good as mine.

They most likely have a clause in the service contract that will allow them to cancel you for repeat overage. I know that some wireless ISPs will send you a warning letter after repeat data overage, then cancel your service after X number of violations.

Oh, and it is free. Both types of free.

...would be Pandion. It only supports XMPP/Jabber, so you wouldn't have to worry about outside clients quite as much(Gtalk could still be a problem, but IMOA an easily solvable one). The major benefit of using Pandion is that it "automatically encrypts your connection to XMPP servers." Considering the sensitive nature of the data that will be transfered via your IM system, this is a major benefit. It's also extensible through the use of plugins. Hope this helps.

Here, here!

So it will be out of beta, "When it's finished"?

I feel your pain.