So if you use latex with plaster, the idea is that you want the plaster to actually absorb the water in the latex, allowing it to form a nice skin. If you use polyurethane, the stuff is super sticky, so you need a well-released waxy surface to facilitate pulling the mold away.
Latex doesn't work too terribly well in a silicone mold, because latex needs a way for the water to leave the rubber for it to cure. So if you go with a silicone mold, you should probably cast with polyurethane.
Making a silicone brush-on mold with a plaster mother-mold is a great way to save on silicone costs. You just need to make sure the silicone you use is thixotropic, which is a fancy word meaning it stays put wherever you brush it. You can make a regular silicone thixotropic either by adding special silicone thickening agents, or by mixing in colloidal silica (cabosil). But silica is sorta nasty stuff. Wear gloves and a good dust mask and mix it thoroughly in small batches until it has a consistency slightly thinner than mayonnaise. And do this before mixing part A and part B, in equal proportions to both parts. Brush-on silicones also usually have a quick cure time; under an hour or so. Brush the silicone onto the clay sculpt, not to the inside of a plaster mold.
Metal powder will properly cling to the inside of a silicone mold. It should cling to a plaster mold too. If it doesn't, mix up some dishsoap with a touch of water and brush it on to the inside of the mold before applying the metal powder.