I am interested in putting they factory S 18's on my 2007 G35X however from what I am reading they need to be the same size all the way around.

The staggered wheels are 18x7.5 and 18x8.5. Has anyone just put the 18x8.5's on all 4 corners? will they fit?

Yes they'll work just fine but you've confused yourself reading about the X. Wheel size doesn't really matter, it's the tire size that's important. Keep the diameter of the front and rear tires within 3% of each other and you can run any wheel/tire combo you want. Use this tire calculator for a quick comparison of different tire sizes. http://www.miata.net/garage/tirecalc.html

So for example I'm running 235/45/18F and 245/45/18R. The fronts are 26.3" diameter and the rears are 26.7 which is only a 1.5% difference. This principle applies to RWD and AWD Gs.