Player skills should matter for good. But we believe PvE will be designed for cooperation of all different classes in Tera, so we do not suppose they need to go anywhere far with party system. We think that classes should always supplement each other in different situations and varied parties should have a slight tactical advantage against random parties.
We will have nice balanced numbers of every class characters in that case. So classical make up should work the best, from our point of view. There's always a possibility to try taking those raids out with no tank around and no one to heal you a bit If you want to experiment(tera gold), go ahead and do it it will be possible for skilled players and thinkers in most cases, we believe.