The resistance transport has a red coordinate action. Leia crew adds a purple coordinate to your action bar. Does that mean I get to choose, when I coordinate, if I want to take a stress for it or spend a Force?

Yes. Leia adds a purple coordinate to your action bar, so a Resistance transport with Leia crew would have both a red coordinate and a purple coordinate.

Note that Tactical Officer and Engine Upgrade work the same way with white actions; the new action is added, but the old action remains available. A YT-1300 with Engine Upgrade could still choose to perform a red boost if it wanted to for some reason.

EDIT - also important to note that you can still only perform a given action once per turn. A Resistance transport with Leia crew can perform a red coordinate or a purple coordinate, but not both in the same round.