I have six Macs, four with SSD and two with FD. My main editing computers are a 2015 iMac 27 with 1TB SSD and a 2013 iMac 27 with 3TB Fusion Drive. All my media is on a Pegasus Thunderbolt RAID array.
While SSD is much faster in I/O benchmarks, in general there is not a huge difference in FCP X editing performance if your media is external. In fact the editor of the Will Smith movie "Focus" used an iMac 27 with 3TB Fusion Drive, with media on a 32TB Thunderbolt RAID array.
On larger multicam HD productions, or if acquisition is in ProRes, raw or 4k, the media won't remotely fit on a 1TB SSD so you must use external storage anyway. If you must use external there's a good argument for SSD. It can be consistently faster for loading apps, rebooting, updates, etc -- vs a Fusion Drive with a fluctuating cache state. The extra storage space of FD encourages use of that, yet you generally don't want media on the boot drive. As FD fills up it will gradually slow down since the 128GB SSD has more trouble retaining frequently used data. However unlike a pure HDD, Fusion Drive performance does not fall off a cliff as it approaches 90-95% full.
I personally would not partition a FD to be separate 128GB SSD and HDD partitions. It works pretty well in full automatic mode. The big benefit is the self-managing nature and it's a lot faster than a plain HDD.
Re noise, space, etc of an external HDD, the 4TB Seagate Backup Plus Fast is a USB 3 bus-powered drive that is very small and requires no external power. It is pretty fast -- about 200 to 250 MB/sec. It is internally RAID 0 so you'd need to ensure good backups.
I am satisfied with FCP X performance on my iMac with 3TB Fusion Drive, however my new iMac has SSD.