Z500
Yes, you can mix firmwares. The only condition is MODEM and Rsrc1 be from the same firmware.
I don't recommend to flash neither of Boot files. You can use files from Z300.
For example, my actual firmware is a mix
Z300R3X01 Modem, rsrc1, rsrc2
Z500BUEG3 Filesys (due to include several games in demo version: Prince of persia, snowboard, ...)
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