To my understanding you will need atleast Security Plus license on the ASAs to get Active/Standby Failover working.
Otherwise if you are running software 8.3 or newer, only 1 of the devices needs the License to get some feature. I'm not sure what the current situation is since it seems every time I try to check something related to Licenses, something has changed (to be honest I dont have to worry much about licensing myself in my work)
Also to my understanding the additional modules on the ASA will need to be kept in identical configurations manually as there isnt any kind of automatic configuration replication between the modules.
Heres some documentation related to the different ASA5510 setups from Cisco site
- At the bottom of the document there is a table of different ASA5510 setups and licenses