That's the cool thing about bond methods other than 802.3ad: for a single interface, the other side doesn't have to do anything different at all.
add bonding group 5
set bonding group 5 mode active-backup
add bonding group 5 interface eth5
Now you use bond5 on the firewall instead of eth5, and you can change which physical interface backs bond5 later. So if you find ElasticXL wants to use eth5 for sync, you just do something like this:
add bonding group 5 interface eth8
delete bonding group 5 interface eth5
and move the cable on the firewall. Now eth5 is free to be the sync interface. Subinterfaces on the firewall will all reference the bond instead of eth5, so you don't need to make any changes to them. Cluster VIPs also reference the bond, so again, no changes. If you move to different hardware later which doesn't have an eth5 or an eth8, you just change the config to reference eth1-07 or enx78e7d1ea46da or whatever the new name is.
From the outside, it all looks like you're using the interfaces directly. The switch team doesn't need to do anything different at all until you want to add more than one link.