I would get a 6" plate, a 10" plate, and 8 of the long legs you cut to size as needed for each cake+separator height. Press the plates onto the styro separators to mark where the columns will go, and make holes in the styro before trying to push the legs through. This will also stop pushing bits of styro into the cake below.
So, 14" cake has 9" styro separator on top. Put 10" plate and long legs on top of the 9" separator which you have royal icing'ed to the 14" cake.
10" cake has a 5" styro separator on top. Put 6" plate and long legs on top of the 5" separator which you have royal icing'ed to the 10" cake.
Don't try and use an 8" and 4" separator - there isn't enough space to fit the legs in, you need a slightly wider diameter. I just measured the distance diagonally between legs of the 6" plate and it is 4" - so if you tried a 4" styro separator under the 6", the legs would not go into the styro. Am I making any sense?!
Each cake and styrofoam spacer on a board the same size, support system under each foam spacer... similar to this:
thanks I think I got it