Good morning
I'm doing a wedding cake this weekend and there will be 3 tiers (10, 8 and 6) the 10" on the bottom and 6" on the top will be 4" high however the 8" in the middle will be 8" high. I'm worried about it sinking in to the bottom tier.

Should I use a plate support system for this cake or will it be ok with dowels only? The bottom cake is fairly dense (almond butter cake) and the center and top are lighter, but the double height has me concerned.

There should be supports and dowels in each of you cakes (not the top cake, obviously) and all tiers rest on those and are NOT held up by the cake below itself. I would treat the 8" middle tier as two cakes (support and dowels at the 4" mark) and then just ice them as one. The general rule of thumb is supports for every 4" of cake.

I've also heard of people putting extra cake boards in between an extra tall tier so that you don't have an 8" slice of cake to put on a plate. You can put one every 2 layers.

This is referred to as double barrelling, see here for a basic explanation: http://www.flickr.com/photos/deliciously_decadent/sets/72157621850372685/with/3798619197/