To answer your other questions:
It is not a good idea to run different firmwares on the members of a HA pair. Especially, not different Major versions (6.2 Vs 6.3). This may lead to config sync issues, session sync may fail etc., There is also a small chance of CPU utilisation spiking up on the primary device over a period of time.
Also, in case you have enabled 'DHCP server' option on the FW, it would be a good idea to turn it off before upgrade and turn it back on after the upgrade. There is a related defect that gets triggered during an upgrade and might leave the FW in a reboot loop.