For example the action card wookie Guardian, does chewie have to be to be present in the system to use that card or can you just play it regardless of where he is located, or vaders action card where he can capture another leader that was just rescued in the same system or something to that effect

They need to be there

RR Pg 2 "Action cards used during a mission or combat can only be used if one of the leaders shown on the card is already in the system in which the mission or combat is occurring. The only exceptions are action cards that specifically move the leader to the system."