"Most road and racing bicycles today use 622 mm diameter (700C) rims, though 650C rims are popular with smaller riders and triathletes. The 650C size has the ISO diameter size of 571 mm. Size 650B is 584 mm and 650A is 590 mm" (Wiki)
It's just weird to see 700c and 27" being used to refer to the same size wheel. 29" mountain bike rims are the same diameter as 700c road bike rims which some people call 28", 27.5" mountain bike rims are the same diameter as 650b, but wider so they fit MTB tyres. Confused yet?
I think there's little point in getting a wheel built specially with 12g spokes. 13g fits the holes in the hub flanges perfectly. 14g rolled threads fit through the holes in the hub flanges perfectly; and 15g deflects less at the third cross.
Afaik, only the DT Swiss Alpine III spokes are triple butted 13g-15g-14g. (But are you doing the Megavalanche? lol)
Plain gauge DT Swiss or Sapim stainless steel 14g spokes are already pretty damn strong as long as they're properly tensioned and undamaged.
The strength of a bicycle wheel depends on getting a lot of tension into the spokes. Wire spokes twist too much to get the tension needed to build a really strong wheel.