Wing/centre/then its more interchangeable depending on the situation and defensive/attacking ability's but it's usually half backs or loose forward, with second rows props hooker down the middle with fullback shadowing play roughly 30m back.
As for Rush/drift defence well it's probably more a case of numbering up, making sure you have the same numbers on each side of the PTB as the opposition, it's more man to man marking and if your winning the collisions and getting your defence back ready you will then rush up but like in Union if your loosing the collisions defences get sucked in.
When you are short of numbers on one side the temptation is to drift but you really need to be more pro active because backline attacks are well structured to exploit overlaps and stop drift defence.
In this situation good defensive centres will try to read who the ball is going to and rush out and smash them, a winger will come in if the centre comes in and move out if the centre moves out.( I would prefer wingers to hold their chalk more and just make the decision to come in or not at the last second)