Yes in tango you tend to place your feet already turned rather than turn as you come onto them in the swing dances.

Knees tending towards each other is an important component of all the standard/smooth dances, but it is most pronounced in tango. Even something like the back half of a waltz turn with it's pointing alignment that we may think of as having the hips very open still has a character of knees tending together. That's in part because the pointing alignment doesn't happen as a static point, it's a succesions of positions one moves through. Something to think about: generally the degree of flex is fairly symmetric between both knees - they bend together and they straighten together. That's in actual dancing - if we try to demonstrate some of the position we pass through in the course of dancing, we may find we have to bend out knees different amounts in order to be comfortable and stable without moving, but that's not really what happens when you dance.