I am also seeing that many of these bikes have thicker spokes on the rear. The spokes are matched to the nipples, and few nipples are cross compatible. Every size has a specific nipple, and many brands have their own threading pitch.
12ga will work across the board from what I can tell, but some rear ends may use 11 or 10ga. 11ga will probably slip fit into most of these wheels too, maybe a bit of drilling the hub and rim. The threads are actually bigger than the spoke, so that is the critical point to pass through.
Once I get the first batch in I will be able to tell you how the 11ga fits into a puch wheel that is meant for 12ga, best that I can tell and measure.