The question covers a lot of territory- what do you want to mold, what do you want to cast copies out of, what do the castings have to do? For example, if you're molding a glass bottle, you'd better be sure to coat it with a clear gloss, otherwise silicone will bond to it. Then, if you want to cast clear copies of the bottle, I'd recommend using a platinum silicone, as you don't have to bake it out for (some ) clear urethanes. Rubber (latex) mask making is a whole different animal, requiring stone (plaster) molds to absorb excess water/allow the latex to accumulate on the surface, and is water based, not silicone, needing acrylic/water based pigments, etc.