No issue with multiple drives with different systems, I've installed first Windows 7 and Windows 10 each on a SSD.
I installed in my Mac Pro (the easiest way) macOS Sierra on a SSD, then the Nvidia web drivers, then I installed Clover EFI Bootloader on the EFI partition of the Sierra-SSD. I mounted the the EFI partition with Clover Configurator and replaced the folders in the EFI-partition with ASRock Fatal1ty Z270 EFI, and activated the Nvidia web drivers (In Clover under 'Sections' check 'nvda_drv=1').
Then I updated some files according this guide: http://hackintosher.com/guides/updating-hackintosh-sierra-10-12-6/
Only then I installed the SSD in the PC, macOS up and running.
Great to hear, thanks for the great info!
--rob