I’ve not made a cake nearly as fancy as those pictured, but to figure out scale I usually stack up cake dummies in the sizes I will be using then I can visualize sizes and scale. I would start modelling months in advance. You can keep them when they are dry. Store them in a dark place to preserve colour. They can be attached to the cake easily if you build the characters with internal support that extends out the bottom. That is another reason I build on cake dummies they are foam and I can push skewers and figure supports into the dummy. A good wet sticky piece of fondant under the characters will also help them stick to the finished cake. I do that when I need a little extra support.
Hope that makes sense and is at least a bit helpful. I would ask one the people who do this type of cake often for tips and pointers.
Good Luck
Teri, Ontario, Canada http://www.TeriLovesCake.ca
