How to avoid clash of multiple RF modules [315/433 Mhz]?
How can I make sure that particular pair of RF doesn't clash/interfere
  with other pair.


Send as little data as possible in as short a time as possible and live with inevitable transmission clashes by making timing gaps between transmissions pseudo random. Else put a receiver with the transmitter and use a single-master to multiple-slave topology.


  I have read that upto 500 different channels can be selected which
  will avoid the interference, but couldn't find much info on that.


Those modules you have selected are not capable of being reliably used with different carrier frequencies.

Will it work? It all boils down to how little data needs to be sent and how often you can live with corruption due to statistical clashes.