Broseph,
FACT: The sizes you have picked for your G are weak.
When you get tired of that set and feel the need to better fill out the rear wheels (I'm assuming you're talking about being flush) look into some 19's or if you want to ball out of control, 20's.
As far as width and offset; it's up to you, but you could have gone bigger in size. The ideal offset needed to pull off a decent flush look would lie anywhere from high teens to mid twenties (ex. +18 to +22). I'm not saying you HAVE TO have those offsets, but I have seen a lot of G owners get the best results with those sizes. If you want to get picky, do a search for "offsets" and "wheel sizes" because I vaguely remember seeing a thread dedicated to determining the sizes you want/need.
Here is a link to a thread by a fellow NICO member who set his sedan up nicely:
http://forums.g35club.org/zerothread/445691
If that is what you are aiming for, doing what I suggested will put you in the right direction.
Good luck. Post pictures of it regardless
And if you don't mind me asking, what kind of wheel did you buy?