My only question now is: Is there a big disadvantage in ordering a bridge pickup now and the neck later when I can afford it vs buying them as a calibrated set?
There's no big disadvantage, a bridge Mule and a neck Mule are basically standard pickups.
But.... when they're putting together calibrated sets, I guess if they find a slightly under (or over) wound bridge model they'll match it with a correspondingly under (or over) wound neck model.
Your warranty card will show the magnet and DC resistance of your bridge pickup, so you can always mention that when ordering a neck pickup later.
