If both the upper and the BCG are new I'd say there are no worries here, (as many folks that make uppers purchase their BCGs to place in their new upper from other sources) that said here's my opinion.
While mixing and matchin BCGs is generally not a problem there are many around here, (myself included) that do not adhere to this philosphy.
I buy my new uppers, (whatever style or brand) with new bc/b/ch s and these part remain with that upper for as long as they are in my possession.
New parts such as the BCGs begin to take a set the moment the rifle is used and these part wear "together" with the barrel extension on the upper. After thousands of rounds the parts are mated IMO and I don't seperate them even if I decide to sell the upper, for it provides somewhat of a guarantee that the new owner will be happy with the upper as I'll know it runs without needing to have the headspace checked and don't have to worry about the "suspect" parts placed in the upper should it be sold without the BCG it came with...
YMMV, but I won't buy a used upper from anyone without the b/bc/ch that came with it new, as those that hang on to these parts for whatever reason don't share my philosophy of keeping working parts together.
ps - fwiw, even though my opinion above precludes the need for headspace gauges, they are some of the first parts I purchased and keep handy in my spare parts kits, folks that believe in mixing and matching used parts should have set too, or at the very least have a "field" guage
ETA - forgot to mention, the bolt carrier is NOT the part with issues here and swapping them around will create no issues whatsoever as it is the "bolt" that actually mates to the barrel extension, that said I don't normally mix and match my bolts and carriers, but do have several new spare bolts in my parts kits, just incase I have a bolt failure. Sould that happen I'd have no issues with dropping a new bolt into one of my uppers, checking the headspace and continuing on