I am making my first wedding cake next month and the bride wants the bottom 2 tiers (14" & 10") to be fake cakes with a 6" real cake for the top tier. All tiers will be be 4" high & covered in fondant. I have a few questions about this.

1. Do I need to use dowels to support the fake cakes or is the Styrofoam strong enough on its own? Dowels to support small figurines?

2. Do I need to use royal icing under the fondant of the fake cakes (I plan to make them 1-2 weeks in advance) or is buttercream ok to use?

Thanks for the advice!

I did a three tiered cake last month that had the bottom 2 tiers (12 and 9 inch) Styrofoam covered in fondant  and I didn't do any sort of support system between the two tiers.  But I also didn't take it to the venue fully assembled.  

I stacked the two fake tiers, with melted chocolate between the tiers to hold them together.   Royal icing would work fine. I normally use buttercream, but like you I asked questions on here and someone suggested chocolate so I did it.  I added the top tier at the venue with buttercream to stick it to the 2nd tier.

Hope this helps!

So it's a 3 tier cake--14, 10, 6--4" tall tiers with the 2 largest tiers being styrofoam.

No, no supports needed between the 2 fake tiers and no support under the 6".  Yes, support in the 6" for any topper.  I use wide bore straws for this [bubble tea, milkshake type, etc.]

No need to cover the styro with any icing before applying the fondant.  Make sure that the upper edges are softened so that they don't cut the fondant.  You can do that by rolling the edge on the counter, sanding them down, or rubbing a small rolling pin over them.  Rub the dummies all over liberally with shortening [you can also use that to fill in any holes in the styro] and apply the fondant.

You can stick the dummies together with royal, melted chocolate, watered down fondant, or even low temp hot glue.

Personally, I'd stack it at home and transport it finished.  It'll be very light and easy to manage.  With the real 6", it can easily be attached to the dummy stack with any of the glues I mentioned above.

No support for the dummies, and I use bc Icing to hold my dummies together. other than a cake round under the 6" you don't need support for it.

I wouldn't use supports in the dummies. I usually use apricot glaze (nappage) to stick the fondant to the dummies but shortening would work well too.