I have been asked to make a cake (very short notice) with mickey mouse clubhouse on top i was thinking i could make the clubhouse itself out of RKT but have never used them! if i make to clubhouse out of these can i then just sit this ontop of a fondant covered cake or do i have to put a board under and dowel it for support??
any advice greatly appreciated

I think you should put a board under. I'm not sure about the dowels for support. Probably depends on the site.

Here are two links to RKT. You might have to use modeling chocolate to smooth it out so that it's not bumpy.

http://sugarsweetcakesandtreats.blogspot.com/2010/11/toy-story-cake-and-cupcakes.html

http://sugarsweetcakesandtreats.blogspot.com/2010/08/barney-and-friends-cake.html