Big Sky is flying into Seattle as a result of taking over the Moses Lake-Seattle run that Horizon used to handle. The route had been unprofitable for Horizon for several years, so they asked the DOT to seek another carrier to take it over. Big Sky took over the run back in July, and I believe it's a subsidized route under the EAS program.
As far as I know, Big Sky is still an all-Metro fleet, and I haven't heard of any plans for acquiring any other type of aircraft.