i want to stack a high "real" cake and then 4" dummies in between each to stick gp flower in....how do i go about stacking structure...for sps.. does the dummies need support inside or not?

Not. The dummies can support the weight of the cake.

When using support in your real cakes, the dummy will be on top of the SPS, however, if you aren't using a single-plate separator, make sure each dummy is on its own cardboard so that they aren't pierced by dowels.