[Neithan]

They would simply have to do so in bodily form, and the Elves who dwelt there (outside Mandos' Halls) were perfectly content to do so in such form.

I still think that because of the Istari's special circumstances they would not be bound to their forms after they came back to Valinor.

But the fact that he could no longer take fair form certainly implies, to me at least, that he could take other forms.

Not necessarily, it could just mean that when he was taking physical form again after being destroyed the form he took could not be fair. Also in the quote provided by HerenIstarion it says,

"It is said that the longer and the more the same hröa is used, the greater is the bond of habit, and the less do the 'self-arrayed' desire to leave it. As raiment may soon cease to be adornment, and becomes (as is said in the tongues of both Elves and Men) a 'habit', a customary garb. (italics mine)

So you see when a Maia was bound to a hröa they were bound to one specific hröa. As for the Balrogs, if they could still shift shape then I am sure they would have done so to escape falling to their deaths.

And why should he have been any different from other Maiar?

Well, since he put a large portion of his power into the one ring, that power stayed intact when his hröa was destroyed. This, I believe, is the only reason he was able to come back the last time. If the ring allowed him to come back, then it could have allowed him to shape shift as well, but personally, I don't think so.