Librem Mini wasn’t based on NitroPC 1 (nor was Mini v2 based on NitroPC v2, etc.), as far as I know. While they share board designs, as far as I can tell the Minis came first, and the respective NitroPCs ship Mini firmware modified with Nitrokey’s configuration. (See e.g. the NitroPC firmware builder, the NitroPC v2 configuration is CONFIG_BOARD_PURISM_LIBREM_MINI_V2 with the vendor name set to Nitrokey, etc.: coreboot-builder/nitropc-defconfig at main · Nitrokey/coreboot-builder · GitHub)
I can’t say for sure whether it will work. Designs like this often have many options and can be populated in different ways. Some of the obvious ones you can see in the photo are the optional COM port headers, the option for a second Ethernet interface instead of the rear USB 2.0 ports - but those are just the obvious things. The design may support alternative parts for any number of internal components that wouldn’t impact end-user functionality, but allow the vendor to adapt to fluctuating component availability. The EC firmware may change, possibly requiring corresponding changes in the system firmware. If so, it should come with matching vendor firmware, but work in coreboot might be needed to interact with the EC in the right way.
When we ship devices, we ensure that everything works, either by ensuring they have the same components where it matters or by adding the necessary support to coreboot for the variations that we have shipped. For example, Librem 14 recently gained support for new Comet Lake steppings, and Librem 11 added a second set of HDA verbs for an alternate audio codec.
So I can’t know for sure whether a device from another vendor will work with our firmware. You’re always welcome to try it, and contributions upstream to coreboot to support more variations are appreciated.