There is no difference between American and Int'l Style technique in Tango. There are some different figures (as well as some slightly different interpretations of similar figures), but no difference in the actual technique. With regards to opening or closing your feet when preparing PP, the answer is: It depends on the figure. There are examples of open and closed feet entires to PP in both American and Int'l style.
One thing that the American style has that the Int'l style does not is the Tango Close. Many people dance an alternative version of the Tango close which turns to PP. This version can end either with the feet open or closed, depending on your own personal taste. I tend to think that closing your feet while turning to PP produces a weak movement for the Tango, but it seems to be the preferred method for social dancers. If you really want to characterize the Tango or stick with a more competitive technique, I'd skip the third step of the Tango Close and just turn to PP. Of course, at this point, calling it a 'Tango Close' would be a misnomer, since there's no longer a 'close'.
As to how much the various parts of the body turn, it depends again on the figure. But with the figure described above, basically each partner should turn their bodies approximately equally: Feet turn 1/8 (or slightly more), frame and torso not at all, and thighs and hips somewhere in between. The head, of course, turns as far as possible, s long as the head axis can remain vertical.
Sincerely,
Jonathan Atkinson