Pilots CAN still fly for the smaller carriers that have 19 seats or less, as they are able to operate under regulations called -135, which are less stringent than -121 (-121 is Delta, United, Freedom, Colgan, SkyWest, etc.).
The maximum seats for scheduled air service under FAR 135 is 9 or less. Air carriers operating 19-seat aircraft must be certificated under FAR 121. This went into effect in 1997.