You will not be able to make an HA pair out of these two firewalls as they have different interface 'types.' The interface type is denoted by the 'entrytype' key in the 'cf interface query' output. For HA pairs you can have entrytype=interface (standard interface), vlan, or nic_group (LAGG). The entrytypes must match on each firewall so that the primary can push its configuration over to the secondary; it cannot push the Cluster IP information over to a secondary which does not have the same types of interfaces that it has itself.
You must turn off 'Link monitoring' for any HA interfaces which are not being used (e.g. interfaces that are not plugged in). If a secondary has an interface that is unplugged but has link monitoring enabled then that firewall will not take over if the primary goes down.
Thanks, that answered my questions.