Not completely sure I understand what you mean by them 'not being active'. They're all in Lightroom and I never know when I might go back to edit a shot.
In a way I guess there is a point about space: if I'm going to end up using external drives, then why not get a smaller internal SSD to begin with? But as I understand it,
SSDs are faster when they're larger, and in any case I guess a larger internal space to begin with is just easier....
Jenny, you are correct. A key purpose of using cataloging software like Lightroom and keywording your photos is to expedite future searches. You obviously can't do that easily if the photos are off line because you selected a small external SSD.
It is true you can use LR to move photos to a different hard drive, and they stay cataloged. However this is a hassle because LR is not designed as a file manager. IMO it's a headache to keep moving files back and forth because of using too-small SSD storage to obtain questionable real-world performance benefits.
The key point is whether the additional *benchmark* performance of the smaller, more expensive external SSD storage has any real-world benefit for your workflow. If not then you paid more money for smaller storage and a more complicated workflow without obtaining any improvement.
Most of the time-consuming actions in H264 video editing and photo editing are not I/O bound but CPU or GPU bound. Infinitely fast I/O will not help because it's already bottlenecked on the CPU or GPU and there's nothing you can do about that. Anyone can see this by monitoring with Activity Monitor or iStat Menus while doing common operations like LR import, 1:1 preview generation, export, or similar video editing tasks.
Part of this depends on your type of imaging work, budget and current/future space requirements. Using cameras like the Sony A7RII and Nikon D810, my documentary team can easily shoot 100 gigabytes per *day* of raw stills. A significant body of work just won't fit on economically affordable SSDs. If your projected shooting volume is far less, then you can get by with less storage. If you will ever be shooting 4K video, that takes a lot of space.
I have a 2013 top-spec iMac 27 with 3TB Fusion Drive and a 2015 top-spec iMac 27 with 1TB SSD, both with Thunderbolt RAID arrays. Media is on the external drive and LR/FCPX catalogs are usually on the internal storage. I can't see any significant real-world performance difference attributable to I/O. On the 2015 iMac I could get by with a 512GB SSD but got 1TB just to give more "elbow room" so I didn't have to manage it as closely.
If you will use Boot Camp or Parallels, that can take up additional space on the system drive, which could argue for a larger SSD boot drive.
I don't think there is any I/O performance difference between the 512GB and 1TB SSD in a 2015 iMac. That other info does not reflect the 2015 iMac's advanced SSD technology.
If you can afford it and the space of external SSD fits your needs, there's nothing wrong with that, but you probably won't see real-world performance advantages vs a conventional RAID array. The SSD may be quieter and might have statistically better reliability but SSD can fail and must be backed up just like a HDD.
From a budgetary standpoint you must remember the need for backup, which scales upward with the storage used. For serious work, two different backup types are best, e.g, Time Machine on a continuously-connected drive, and Carbon Copy clone backups on a periodically-connected drive. Doing that triples the storage requirement.
