leopena_94:
First I changed the RFID's SDA and RESET not the I2C if I am not mistaken. I changed the pins for SDA from 53 to 8.
You are very mistaken. SDA refers to the i2c bus and is on 20,21, as I said before. If you are using pin 8 for SS of the RFID, that sounds fair enuff, so long as the SS is connected to pin 8 If you are trying to select SD on your Ethernet shield, try pin 4
As I have read in this article https://www.arduino.cc/en/Main/ArduinoEthernetShieldV1, pin 53 must be left unused or the SPI would not work.
No it doesn't, it says
it must be kept as an output or the SPI interface won't work.
If you are using the current IDE, you can ignore this anyway. You will see this, it's absence, in the examples in the SD section of the IDE.
I'm afraid I can't comment on the library. It should be OK if RFID runs on SPI. I think your immediate problem is that you don't know the difference between I2C and SPI.