It is best practice to run HA cage with even drive counts across enclosures if you can swing the purchase of the SSDs. HA cage and identical drive counts across all cages are not required, just best practice.
The main requirement is that unless you want major issues you must always keep the count of drives matched behind
controller pairs. One new enclosure would connect off of the Node 0/1 loops and the other off of the Node 2/3 loops and you would be balanced with 80FC and 16SSD disks behind Nodes 0/1 and 80FC and 16SSD disks behind Nodes 2/3. Disks must be added in pairs, adding 2 behind Nodes 0/1 and 2 behind Nodes 3/4, with a minimum of 8 per type per node pair unless they are cache drives then you just need 4 per pair.
if you aren't ready to add the expense of SSDs, you could add the 2 enclosures with 20 FC disks each and leave the remaining 4 slots open in each for future SSD expansion. Then you can stay at HA cage for both SSD and FC if wanted. You would still be balanced overall with 80FC and 12SSD disks behind 0/1 and 80FC and 12SSD disks behind 2/3.
If you are running HA cage you should have the same drive count for the drive type (10k/15k/100k, etc) in all
enclosures that drive type is present in. Just as you can mix LFF and SFF enclosures you can have enclosures with SSDs and without. That said, you really want to spread the load as evenly as possible, and nothing is more even than the same disk counts in each enclosure. Did I mention that the drives must always be balanced behind the node pairs?
The other option is always to add 6 enclosures, another 120 FC disks and move to 2 SSDs per enclosure
