Is it possible to mix memory? Yes, no, maybe. If mixed, they aren't identical.
Still it is possible, but it really depends on the chosen memory.
Why are you considering 128GB or 96GB of memory? For what use?
Trolling? Nope.Honestly? No trolling please, for looks, also for RGB, i dont want "holes" between my dimms and rather have all the slots filled, I have G.Skill NEO
If this is just for aesthetics, I think gskill (and maybe others) make dummy RAM sticks. Could install 4 real sticks and fill the rest with dummy sticks.
Edit: It was Corsair, not gskill.
Trolling? Nope.
Just an awful lot of money to spend for memory you won't typically use for aesthetics.