Precision rack 7910 memory upgrade

Good morning, I have a dual CPU precision rack 7910 workstation that has got 32Gb of memory organized in this way:

- 8x(4GB 2400Mhz single rank x8 DDR-4 RDIMM) modules working at 1866 MT/s populated in all white slots (A1, A2, A3, A4, B1, B2, B3, B4)

I would like to upgrade memory to 64GB or 96GB and I found these modules:

- 2 or 4 x(16Gb 2400Mhz dual rank DDR-4 ECC RDIMM)

As explained in the manual I should move the 4Gb DIMM on the black slots and install the two bigger 16Gb dual-rank modules (mirroring memory configuration per CPU) on the white slots. As buying other 4 modules to completely populate the white slots will be expensive, can I leave part of the white slots unpopulated and leave all the 4GB modules on the black ones? If it works, what will be the performance impact of leaving part of the white modules unpopulated?

I have also another question, can 2666Mhz DDR4 ECC RDIMM modules work (clearly limited by the 1866/2133 Mhz specification) on the precision 7910 rack workstation?

Thank you

Matteo