[Galadriel55]

I think that when the Istari came to ME, Gandalf already seemed the oldest. As for the legal-or-not thing, Gandalf is the "stewart" of all of ME, and thus should have the authority to decide what will happen to Isengard. He has more authority than Aragorn or Denethor or both combined.
If the staffs are symbols of the order, then what do the colours of the wizards signify? I thought that colour has a corresponding rank to it, white being the highest. Saruman "renounced" his colour, but wanted to keep the rank assosiated with it. Maybe by breaking his staff Gandalf showed him that its either none or both.
I find it curious how Gandalf only has the power/authority to break Saruman's staff and kick him out of the order when he becomes white. It could be because until that time Saruman was still superior to Gandalf, no matter how corrupted he got. It is possible that when Gandalf was sent back he received more power/authority that Saruman had, or at least the same amount. An inferior can't kick out his boss, but an equal can kick out an equal.