This video explains the simple task of rescuing units via the trigger editor. If you'd like to see other video basics, let me know. If you'd like something advanced, then only choose it if you've seen it in one of my maps :P
If SC2 is anything like WC3 (and I know it is since it's practically the same engine), you can treat a unit as a variable and reference that variable for other triggers. So if the player built a Marauder from a Barracks, and you wanted that Marauder to be the one to rescue the units, then the trigger would look something like 'Unit is trained; Unit type = Marauder - Create variable with value [last created unit]' and you would reference that variable in the rescue trigger instead of 'any unit'. Just a hypothesis, though, since I haven't done much with SC2.