miller,
There are only two lines in SLK. One line holder bid Guam and will be leaving, however as the new bid should be published today, that open slot will most likely be filled before you get on line.
navi,
Mail flying is only HYA-ACK. SIC's could be based anywhere, but most flying is out of HYA and BOS. Remember the SIC is not required VFR, and replaceable with an autopilot under IFR. Thus there is very little need for an FO in the Caribbean. RUT is always 2-person crew route, so I understand there are 2 FOs based up there.