That's correct about the 5-on-5 being the pattern for the 12" rotor only, which is then on it's specific steeering knuckle. Found on full-size cars as an option but getting scarce in the junkyards. They are probably most common on late-'70s Cadillac Sevilles with the 4-wheel disc option...if you see an X-body Seville w/ rear discs, then it has the big front rotors w/ 5-on-5 all around.
Probably you can't turn down a 12" rotor to the smaller size, I believe the center "hub" is also bigger and you can't turn that down too...but you'd have to check. Me, I don't have one handy.
Those brakes, f & rr, are a virtual bolt-in for a 2nd-gen Camaro...if you were lucky enough to come across a set, why not switch 'em all out and get your bolt pattern at the same time? I have that on a '70 El Camino (in which case I had to use a 1LE rotor to get back to the 4 3/4", and slot the rear disc hat holes) and it stops really well. The downsides are, crappy e-brake setup and them are some heavy front knuckles.
Or...re-drill your stockers, if they'll take it and you're happy with your brakes the way they are (that would be the easiest way), however sometimes there's cast-in ribs on the back side which prevent drilling another pattern...those ribs may not be present on replacement rotors, you might ask your parts store to pull some out of the box to see.