There are 2 kinds of Physical Immunity that are relevant, and as far as I can tell, it is only the "always, but..." part that seems to be causing headaches. I suggest simply removing that part. If someone or thing can only be killed by one thing, that sounds to me like a plot device anyway and I think an aspect should be enough here. And yes, that includes PCs. You can still be taken out in a conflict, the power does not remove the stress you take. You can buy toughness powers as well to do that. I like this approach, because it only dictates the taken out result, not the stress intake. And stress does not only mean damage but all kinds of disadvantages that lead you to lose a conflict.
A more expensive full PI would not really accomplish much in my eyes. PI in my eyes has one definite drawback: it is boring. If you put a high price on it, then a character who actually takes it would not have anything else to play with. If you reduce the "impossible to hurt" part to "impossible to kill", things get way more interesting again, but the power is not really worth all that much anymore.
If we do that, we can take the remaining Physical Immunity and make the "+2 if it only protects from one thing" bonus always on and get a [-6] Physical Immunity power against one specific thing with a stacked catch. I would also force this specific thing to be more narrow than a catch on other toughness powers. So "Immunity against fire" would be fine, "immunity against magic" would not.
If you still think is is too cheap, separate the Immunity power from the rest of the toughness powers, so the catches won't stack.
I do not like a fixed list, because it does not take into account the creature itself. An "Immunity to garlic" would cost a werewolf [-0] refresh, but a black court vampire would have to pay in full and it would remove his other catch bonus as well.
PS: Social Immunity probably should not be possible. How could anyone ever be immune to a bad reputation? Resistance to social stress makes sense, but immunity is just weird.
Some sort of mindcontrol comes to mind. It would probably be very limited, but it would be possible.
Or how are you going to hurt Marcone? Everyone knows he's a criminal, so is is going to be immune to attacks in that area. But again, I would suggest a pointer to an appropriate aspect instead of a full blown immunity.