I have finally assembled the brakes. The bike came with low end rt10 rotors which I'm going to change them in the future, but didn't intend to do so right now.

On the shimano website they're listed as compatible with mt520 calipers as long as you use organic pads. However, the rotors are wide tipe, while the caliper uses narrow pads. This means that part of the rotor track, on the inside, is unused by the pads. Will this cause any problem?

EDIT: finally managed to ride with the new brakes. No problems with the rotors as far as I was able to notice. Excellent brakes for 112€, gone back to effortless one finger braking which is really nice.