I know the general uses for Fondant and gumpaste but what about all the others, Pstillage, Modeling Chocolate, and Marzipan. How do I know when to use one of these and what effects they give???

You just have to play around and see which appeals to you. There aren't any rules on mediums. Pastillage and gumpaste will produce a firmer product, candy clay gives pliability, and fondant attaches better, and marzipan adds a flavor variation. For me, I like the 50/50 combo of candy clay and fondant, it allows the pliabily I need for sculpting, slower dry out time, and no attachment problems. JMHO ("