Can I use a Seagate SSHD in my iMac and if so, how do I load OSX (High Sierra) onto it
Yes, you can use a Seagate SSHD on your 2013 iMac. 

Installing a new version of High sierra I would recommend formatting the disk to APFS, which is Apple's new optimised SSD &amp; 'fusion' file format. Everything else will work fine.


The way the SSHD works is the SSD portion is just cached, and the drive itself manages what goes into the cache rather than the OS.


Seagate SSHD had some issues with the newer drive models in older macs, but that is mainly due to being a 3Gb/s SATA connector.

As your late 2013 machine has 6Gb/s SATA, any seagate drive (Firecuda (new model) or momemntus (old model)) will be compatable.

You can read a little bit more into it on the Apple website, if you're interested in in-depth info.