technically the rotor channel in the 26mm calipers (i have them) is near 30mm and the 30mm caliper's rotor channel is closer to 33-34mm. the rotor will fit in the caliper, but if you load it with fresh pads, it won't fit, or will be as if the brakes are applied all the time. i guess you could "mill" the pads down, but then you're just starting out with a shorter lifespan to begin with, unless somehow you could find a pad that already came with a smaller pad face to begin with.

it could come down to playing with the pads on the rotor with it all installed. you could possibly shove it all in there and have the caliper pistons pushed back far enough to let the rotor spin without continued pad pressure, and they'll wear themselves down over time, but i don't know really. i wouldn't do it.

the audi rotor should be fine i think.

i would suggest using mitsubishi 3000gt vr4 rotors with a bracket kit, but those require 17" wheels so that might be out of the range for you.