How do I stack something like this for the bottom tier?

I'm thinking a Styrofoam cylinder under the big bottom tier and just slide in the small cakes around.  I assume there is no weight on the small cakes.  Any pointers?

no, you got it-- unless you need more servings--

i used the either 3/4" or 1" wooden dowel that i got cut for me at the hardware store--nowadays i would seal them in wax as well but they stand perfectly just on the table so that's what i would use for something like that if it was all cake under there--or maybe use in the white quilted one--

how many servings you doing?

best to you

I doing an 8", 10", 12" and then 4- 8" rounds to serve 160.  I'll use my regular Coast plastic plate system up top, was just wandering about support under the 12"...if additional support needed in the 8" satellite cakes or if the Styrofoam under the 12" was enough to stabilize.  I've actually done this before, but just forgot how I did it!! :(

I've done a couple of cakes set up like that and as I remember I did exactly what you are suggesting.  I think you want to use a 10" styro circle so there is room for the 8"ers to show.  Better yet, using paper or your pans, set up just the four 8s and see what size the hole in the center is going to be - you may not even need any styro - I can't remember since it's been so long ago that I did mine.  You may want to put just one dowel in ea cake on the bottom if you do use styro, depending on how much overlap there is onto the 8s.

i prefer to use 6 inch cakes under a 12"--here's some photos of dummies--

this ^^^ is with six 6 inchers

this ^^^ is with four 8" cakes pulled way apart

this ^^^ is how much room you have in between your 8" tiers --that's a 6" dummy so you could get a 7" dummy in there but it would need to rest on the edges of the bottom boards that the 8" tiers rest on -- 

otherwise you'd stack the 8x10x12 on top of the four 8" tiers -- if you pulled them in together more -- 

AI agree with K8memphis, 6" rounds will look much better than 8" rounds.

The dummies are definitely the way to go. Foam is light so it is easy to work with and wont let you down. I have gotten pre-made dummies from buyfoamblocks.com in the past but I have also made them from scratch using foam discs depending on how big my cake was. I think that using wood could be dangerous because wood is heavier then foam is. 

If you want different sized foam discs to make your own dummy I know buyfoamblocks.com has them and they have pre-made ones. I know how frustrating finding the right foam for the dummy can be, so I hope this helps.