Hi
I start by covering my cake dummies with press and seal or saran wrap... why? so if I want to do another design later on I can just brake the fondant and remove the plastic wrap and I have a dummy cake to re-use if needed....
But just so you know.. it's awesome to work with dummy cakes.. better than regular cake lol cause it's sturdy, some people use piping gel but I use and spritzer (with the settings on the lightest mist you can get) I mist the wrap covered dummy cake and put the fondant... it's very easy... to stack them, since mine are wrapped... I use tan tape (i think it's packing tape) and they stay together....
ALSO....
some people make RKT and mold them into a cake pan to get the shape... and use that as dummy cakes...
Here is a tutorial on it by Lorraine..... this might help you....
http://www.youtube.com/watch#playnext=1&playnext_from=TL&videos=JVZAui19VUI&v=mp1pOvCvjmM
Good Luck!