[jan-jaap]

Could any harm come from leaving it on all the time?

No, it was designed for that. But anything you leave running 24/7 will wear out in the end. My Origin 350 has run 24/7 since I got it in 2011 I believe. Whether you want to leave a 20 year old device running unsupervised in the first place is up to you.

My other question is what's the largest number of disks I can put in this system and what's the maximum capacity of each disk?

The system has space for 3 internal disks. There's a maximum LUN size of 2TB. Actual SCSI disks wit SCA connectors max out at ~ 300GB.

is there any way I could buy an adapter and put an SSD in my Octane? Not so much for performance but for longevity, noise and heat reduction, and more storage.

Well, you'd need a SCSI-to-SATA bridge, one of those Acard devices, except the asking price these days seems to hover around $1000. I think an Octane consumes around 250W so I don't think the ~ 10W consumption of a hard disk will make much of a dent. I'd get myself a late 15K disk with FD bearings , e.g. a Seagate 15K.4. Their noise level is comparable to a SATA disk, except the seeking noises are a little more aggressive.

I'm guessing that partition sizes will be limited by the older XFS versions and possibly the boot firmware not wanting to boot from large disks or partitions. Would just making multiple partitions on one disk work?

No, the only limit is that a single disk (LUN) cannot exceed 2TB. In my Origin 350 I have an LSI SATA adapter with an XVM stripe of two 2TB disks resulting in 4TB scratch space. I could attach many TBs of SAN storage, as long as the SAN serves it up in 2TB chunks, and you assemble them in a single volume using XLV or XVM.