Dnk, in your first question (I am assuming all are copper for this conversation) 2/0 is good.
The second question, you would need to run a GEC from the XO tap to the building steel, the feeders would still need a properly sized EGC (250.122 or one recognized in 250.118) installed with them, and this would isolate from the neutral in the equipment served.
2002 NEC referenced
Edited to add a word.
Roger
[This message has been edited by Roger (edited 07-20-2005).]
