Yeah, that's the stuff.
When you mix the powder into the latex (or resin, or whatever,) it has a strong dulling effect. Aluminum just kinda comes off as grey. On resin, this is ok, because you can buff down a layer revealing the raw metal; but you can't buff latex, so there's not much point. Plus when casting latex into plaster, you want to slushcast the first layer, and if you add aluminum powder, it will thicken it reducing its ability to slush about. Just tint the latex a good solid black. (Some people use a brick red behind brass or gold).
I find the whole layering process too tedious when working with latex. It takes layer after layer to get anything thicker than a Halloween mask. For a bit more money, you can go with a nice thixotropic polyurethane rubber and do the thing in one batch. It's much more adhesive than latex, so with plaster, you want to brush the mold down with a couple coats of alcote (aka liquid foil). Then after the polyurethane has fully cured, you can just dunk the plaster mold in a tub of warm water for awhile, the alcote dissolves, and the casting just floats right off.