1- yes, just connect the HA ports. Best practice says, use redundant HA links, as the worst case of all is when cluster members lose their HA link, and both guess they are master - with identical IP addresses and MACs.
1b- HA monitoring is used to monitor the link status of certain ports (like, the LAN port), in addition to the master device status. The cluster will then fail over if either the master if offline, or one of the monitored links is down.
Link status down is quite seldom, so you can enhance this by letting the FGT check a remote target by ping. If enough pings fail, the cluster will fail over.
But this is enhanced stuff, and I recommend to read up on HA in the FortiOS Handbook before configuring anything.
2- you can use any unused port for a HA link, for example the HA port and port "wan2" or whichever. I specifiy "HA2" as this port's alias, just to remind me. And I use red cables, and warn my customer to never, ever pull these out while the FGTs are running.
"Kernel panic: Aiee, killing interrupt handler!"