The speed you might gain with PCI-e + SATA SSD Fusion drive might not be that significant (given CPU and 4GB memory).

Your Mac Mini is the 1.4 GHz entry level - personally, I would invest all the money to get a Bigger SATA-3 SSD alone for this Mac Mini.

I can see you have spent some energy and money to get the hardwares for this task, however: I know some people would want the newest PCI-e SSD with modification of second SSD by replacing the HDD (2014 Mac Mini Fusion version). The fusion drive is combining the 'speed' of SSD with 'capacity' of HDD. Hope you can help, in any case thanks in advance for your attention. Is there anything I am doing wrong? Is there anything I can do to check if the problem is the connector on motherboard? Should the PCIe port need to somewhat activated? In sum, I tried two different drives with two different connectors but in all cases once I power on my Mac the additional PCIe blade drive isn’t recognised at all as it neither was present nor connected. I bought on eBay a connector 821-00010-A and an original Apple/Samsung MZ-JPV1280/0A3 PCIe blade drive coming from a Macbook Pro then a bought a brand new Transcend Jetdrive 820 especially designed for Mac mini 2014 and finally I bought another 821-00010-A connector from a different seller. I tried to upgrade my Mac mini late 2014 (MGEM2, i5 1.4 GHz, 4 GB RAM, 500 GB HD) to Fusion drive following the procedure posted here and since I cannot succeeded, I am really getting crazy!

Ebay mac mini late 2014