OP wrote above:
"I only use the iMac for video editing & music production... So FCPX, Logic X, Ableton, & various plugins."
Pardon me if I've missed it, but is your iMac the "1tb fusion" model, or the "2tb fusion" model?
The difference is important.
If it's a 1tb fusion model, it has only a 24gb SSD portion (of the fusion drive) inside.
If it's a 2tb fusion model, it has a 128gb SSD and a 2tb HDD.
If it's a 2tb model, I suggest this:
1. Split the fusion drive into separate SSD and HDD
2. Put your OS and apps on the SSD
3. Keep your other files on the HDD (or better -- connect the 256 external SSD via USB3 and use that instead)
If it's a 1tb model, I suggest this:
1. Leave the internal fusion drive alone.
2. Take the external 256gb SSD you have, and create an "external booter". Again, USB3 will be fine. You -can- get a thunderbolt enclosure for it, but the enclosure alone costs around $90.
2a. Put your OS, apps, account, etc onto the external booter
3. Put your music libraries and production files on the internal fusion drive.
Either way above should give you the fastest speeds and fewest "bottlenecks", insofar as data input/output are concerned...