I had 2 sets of 18" wheels already on my C6. PCD 5x112, ET 45-48, Rim width 8J fits perfectly. ET 35, will slightly stick out. I would not advice anything higher than ET 48.
Just be carefull with Center Bore, newer cars has 66.5 CB, older 57.1. I dont know if C6 58 has got 66.5, I can say that...