For the non-starting action cards, do you mix the new and old action cards to form a deck you can draw old or new leaders from? Or do you only use the new action cards and can then only get old leaders from mission cards?

Thanks in advance!

You have to mix all action cards together to form an initial action deck with the old ones and news and draw two from them and also mix the old ones and new ones to form the action deck for recruitment

Also note that a couple of the new action cards at "At Start" action cards (they lack the Recruit symbol). This means there are a couple more things you might draw during set-up.