I actually have an iMac with a 1TB HDD and the tiny 24GB SSD. You can put the OS on it but without the Fusion Drive firmware/software that SSD would crash in a day. It hasn't enough space on it, but in the Fusion Drive framework files can be swapped to the HDD if they aren't frequently needed. One could hardly manage an iMac exclusively from a 24GB drive. The smallest installation I've made is about 50GBs after the OS sets up the sleepimage and the vmfile. Each is equal to the amount of installed RAM.
The 24GB blade is about as fast as most inexpensive SSDs. Top of the line will be 550/530 read/write. Newest Macs now have a PCIe 3.0 bus and controllers that can get the 2200/2000 read/write speeds out of the M.2 nvme SSDs like the 960 EVO Pro from Samsung. My 2015 5K model cannot produce that sort of performance and neither can yours.
I would not recommend trying to create a Fusion Drive with the two SSDs because the slow write speed of the small SSD, 350, will cause the fast SSD to run at that slower speed. I also don't believe the original Fusion Drive is as fast as claimed since the HDD is already a slow 5400 RPM HDD that could not do much better than 100/100 read/write since that is the limit of its transfer rate. I think the test you saw was one in which the caches were enabled on the computer and drives to give the impression they are that fast.
Were I to remove my iMac's HDD I would replace it with a decent SSD, but I would not bother creating a Fusion Drive. It would be a waste. The main SSD would be faster by itself than with iMac's built-in blade.
