Use an ethernet shield and RFID rc522 together on a mega?
Yes, I do think so. according to my Question with RFID and display it turned out that those pins are for serial communiacation. So I think it should work. 
I can verify
 that it will work if you do use the ICSP headers MOSI/MISO/SCLK pins. Connecting those three with the RC552 as well as GND and 3V3. Also connect two other pins to SS and RST. This pins can be adjusted in the sketch on the Arduino.