Some secret rooms, where only selected people have access.

Use Community Builder (important to support CB!) AND/or Joomla's internal groups (registered/author/editor... administrator/super administrator, FRONTEND/BACKEND..) you know... real private rooms.

That kind of rooms require a long chat history as the number of users may be limited.. (referring to the recent "history" topic...

ED: history topic: http://www.blastchat.com/component/option,com_fireboard/Itemid,143/func,view/id,3275/catid,73/

1. Joomla's internal groups will be supported when client 2.3 comes out.
2. I am not sure how community builder has groups implemented, but I will be willing to work on it with you.
3. read topic you refer to, right now history of 25 messages will be printed out if available. (combined active messages + archived messages)