Yes, the rear offsets are considerably different and you'll probably want a spacer.

 

Depends. Base wheels will fit with the proper offsets, but wide body wheels (Grand Sport, Z06, ZR1) won't fit correctly without flared fenders.

 

Will they bolt up to your car? Yes they will.

Will they "fit" properly as defined by the correct term for "fitment" (wheel properly tucked into the fender line)? No.

The suspension geometry for the C6 was changed from the C5, primarily in the rear where the biggest difference is. Base C6 wheels of any year will sit too far inwards on the rear, necessitating a spacer of some sort to get close to "proper fitment." Z06, GS, and ZR1 OEM wheels will sit a little too far out on both the front and rear because on a C6 they are installed on a wide body car. That's not present on the C5, so the tops of the tire's sidewalls are likely to come close to the fender line (it will vary depending on the tire design).

None of this will apply if you buy C6 style wheels from a wheel vendor that has them constructed with C5 offsets in mind. However you need to be careful here. Most vendors utilize a cast construction and while many owners have had decent experiences with cast wheels there are also some horror stories, such as requiring multiple balancing sessions to possibly not being able to balance a particular wheel at all. And most return policies will not allow you to return wheels once they've seen tires mounted to them.