Question!
How much time after the disaster described in the starting post do you envision the story to be set?
I'm happy for your first post to be during the earthquake, if not slightly before. I imagine there will be a little bit of "temporal distortion" (Yeah, that sounds like a good excuse. <img src="/ubbthreads/images/graemlins/tongue.gif" alt="" /> ) in the start of the story until we all catch up to each other.
Other races and monters are fine so long as you justify their exsitance. A mutant race caused by a radium experiment failure (eg. Chernobyl) is a fair enough reason. If you want, bounty hunters could be hired by the government or other private organisation to cover up the blunder.
Another way could be to have a tribe or Magi that evolved their natural senses and appearence to assist with their local way of life. They may or may not be known of by neighbouring tribes or cities.