will there be an option to mix and match alu/poly tops and bottoms?
Likely, but I'll need to verify with KBDfans that it's fine before it runs, since it's quite a bit more effort logistics-side.
Will silver be available? Or is it too unpopular?
Going by KBD8X MKII sales, silver proved too unpopular to be worth over e-white.
Will there be a configurator for the layout to generate the firmware file? I'm assuming the PCB will use QMK.
I'm currently working on QMK for Polaris as I type this message; I'll definitely make it compatible with some GUI-based programming tool, whether it be configurator or VIA (depends on whether or not VIA is finished by the time Polaris ships).