If you have a square stacked cake and want to use the round plates for support (to use with the tubes in the cake ) under the cardboard with the cake on it is this possible if you use rounds smaller than the square? 
Hope this makes sense? Has anybody tried this? 
I would not recommend this. The cake board under the square is not enough to support the corners of the cake that will hang over the edge of the plastic plate. You will see it start to sag. I just helped a friend make a wedding cake with squares cakes on round cakes the the corners hung over the edge and you could see them starting to sag. You could try 3/16" foam core board instead of the cardboard, that might help support the corners more. I would definitely buy the square plastic bases and play it safe! Good luck! 
I'd do it only if the boards were the plastic tuffboards, not the cardboards. The square plated are safer, though!
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