I am making a topsy turvy cake this week with about 4 different colors of MMF. I don't want to make a completely different product (like gumpaste from scratch) and have to try to match the colors to create the curly cues and figures. Can I add gum-tex to MMF to create a gumpaste type substance? Also, most of the cake is a dummy cake. Do I have to ice the dummy with buttercream first or can I somehow glue the MMF right to the dummy cake?

You can add gumtex to fondant to help it harden. I just sprinkle some on. To stick the fondant on the dummy, use a very fine spread of piping gel. If you don't have piping gel, corn syrup and a bit of water will do. Just enough to make the outside of the dummy sticky.