Not sure about the multi-player rankings - can't seem to go up or down too easily. And please tell me they're bringing out more maps soon?!
Once you are ranked, you only go up or down within that rank, i.e. if you're Gold 1, you can get up to Gold 6, but can't go to Platinum, nor can you drop down to silver.
To increase your ranking you must win matches, individual performance means nothing.
And I think each month or so they reset the rankings and you 'qualify' again.
Yes more maps are coming, and they're free.