I haven't used zinc for DMT n-oxide reductions, have only ever used it in harmaline -> THH reductions, but from what I understand you need the same type for both, so can answer some of your questions:
It needs to be zinc metal powder, not a zinc compound like zinc oxide etc.
You want your powder to be fairly fine, but not so fine as to pass through your coffee filters. I use 350+ mesh,
which apparently is ~40 microns. Coffee filters are typically 20 microns.