Hi, i am powering my pixhawk through the servo rail with 4 BECs from the ESCs, if using the BEC from only one of the ESCs, would the motor connected to it spin slower? and I have to replace one of the ESCs, can i use another one, with the only difference being the current rating on the BEC, would this different current rating affect the other BEC connected?
No. The idea behind BEC is to deliver enough power to other circuitry without the need of multiple batteries or components. The motor will not run slower with the BEC connected.
The BEC should supply at least 500mA@5V to allow a stable functionality of the pixhawk, if your BEC can supply that, you are ready to go.
This is not a problem of current rating but voltage, your power supplies (BECs) should have the same voltage if you want to connect all together. Usually this is not necessary, a single BEC should supply enough power for your pixhawk. Also, take in mind that a voltage difference between BECs can damage your system.