This is quite a large question. If we forget about the FWSM for the moment.
"Is SVI required for every VLAN defined on Cat6500 switch"
Not necessarily, it depends. If you want to be able to route to that vlan and from that vlan then yes you will need an SVI for that vlan. If you don't create an SVI you cannot route for this vlan on the MSFC.
So why create just a vlan without an SVI on the MSFC ? Well there are a number of reasons but this is where the FWSM comes in. If you want to firewall a vlan that exists on the 6500 then you cannot have an SVI on the MSFC otherwise it will never be firewalled. So you need to create the interface for that vlan on the FWSM module and not the MSFC.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...