Current network is like - MSP network--> Cisco 4431--> ASA--> N7k--> LAN. I'm adding one more router and one more ASA in the network, Do I really need to add L2 switch to connect routers LAN side and FW outside interface to achieve HSRP redundancy?(as I dont have L2 switch), if L2 is mandatory for HSRP as I went through some docs, how the switchport on L2 and Router ports and ASA ports will be configured? on L2 switch - normal switch port on a single VLAN ? and on Router and ASA normal L3 ports ?
You'll need some kind of L2 switch for the HSRP to work for you in this setup. The switch will give your Routers/ASAs what they need to "share" their Layer 3 address (as Ron Maupin said above). I'm guessing you must have these connections currently setup to run directly from each device to the next with no switch fabric in place?
Keep in mind, while you'll have redundancy at the hardware level for the routers you won't have redundancy in your switch fabric for the ASAs/Routers if you only add a single L2 switch. If the switch goes down, your hardware redundancy doesn't do much for you. Consider adding two layer 2 switches at some point in parallel (A and B side if you will) Just something to think about, but I'm sure you have.
L2 Switch ports - configured with a single VLAN shared for both Routers and the Firewalls.
Ports on ASA/Routers - Same you would have normally configured them except with your Routers using HSRP, and your ASAs setup for HA like Active/Standby failover.
"Do I really need to add L2 switch"
No, you already have one - the Nexus. Simply carve out a few ports into a VLAN for that segment. If you are really concerned that VLAN's do not effectively segment traffic the 7k has Virtual Device Context functionality whereby you can carve out a 'new' L2 switch.