The way the mission logic works, a specific ship must do the rescue.
You can get a better understanding of how to play missions if you study the Mission Editor Notes:
http://80.95.161.114/shipsim/forum/index.php/topic,19793.0.html (http://80.95.161.114/shipsim/forum/index.php/topic,19793.0.html)
The text of the Objective should tell you what craft to use for a rescue. If it does not, then the mission is not acceptable. In that case, you should send a PM to the creator.
You must register your serial key in your forum profile to have access to the Missions board for any further help on missions.
Questions about missions should be asked on the Missions board, following the rules in the bold blue topics on that board.
Regards,
Marty