I haven't sculpted much hair but what i did was always with pure
GS. I don't know what hairstyle you want to create but most hairstyles look best with a more organic flow with strands beeing not all parallel. Just add a strand here or there that breaks with the flow or overlaps another strand.
In my most recent hair i used just normal
GS. I flatened out the the hair cut the strands in with a hobby knife and used a sharp tipped clay shaper to go over each cut again.
You can see a bit of that on those:
As for the mix only add milliput if you want very sharp picked out strands. But hair doesn't need to be perfectly sharp. Even
GW doesn't pick out too much details on hair.
And have you tried out using just milliput standard with
GS like i do? I like it more than mixing with milliput white. It is cheaper and it feels more flexible. (But maybe my white was an old batch

)