Named and Default Instances on single SQL Cluster
What will be the consequences if I create a Named instance on the above existing setup. will the idea work ?


It should work like any other instance. Lot of people have multiple instances of SQL FCI running peacefully.


  do we need separate IP in the same VLAN for named instance and add it to cluster resources ? Also will fail-over happen as before ?


Yes, you would need a separate IP, which will be used for virtual network name. You would also need separate storage for the new SQL FCI. They all can use same network though. Failover should  work just fine and manually you would have control to just failover the FCI you like.