FM receiver used to control a switch
A basic on/off signal sent via FM to a receiver is not going to work. You need to encode a data stream with an address (for each receiver) and incorporate a check sum so that the receiving device can check the address, decode "on" or "off" and then double check correct reception with the check sum.

This means incorporating a little MCU in each receiver and having each MCU have its own address (4 bits sounds adequate for your application). If you think you can do it with just individual frequencies and make it robust to interference forget it.