Not all PCIe drives are supported, thats why we have a list of tested and confirmed working blades/drives.
macOS NVMe support start with Sierra, for 4K/sector drives only, earlier macOS releases don't even see that the drive/blade exists.
High Sierra is the first macOS release to have 3rd party NVMe support for both types of sectorization (512 bytes/sector and 4K bytes/sector), see the PCIe SSD thread first post to understand.
This is a general info thread for blade SSDs that can be used in the Mac Pro. This is a WikiPost so anyone with the proper credentials may edit it. NVMe SSDs can be used as a boot drive in the MP5,1 and MP6,1 with the latest firmware installed (beginning with version 140.0.0.0.0 for MP5,1 and...
forums.macrumors.com
Your questions are already answered there in detail.