Okay, so these are the decided cakes and sizes, no budging on this, as the client really loves this look.

3 cakes total:

1st cake: Single 4" (cake pan size) by 6" tall cake

2nd cake: 2-tier: Bottom: 6" (cake pan size) by 4" tall cake; Top tier: 4"x4" 

3rd cake: 3-tier: Bottom: 8"x4"; Middle tier: 6"x4", Top tier: 4"x4"

Like I said, these are exact, won't budge on sizes, however they only need a total of 40 party servings. They are fine with dummies to achieve this look, but I am not sure what my best bet is as to what cakes or tiers I should use dummies for. I hope this makes sense. 

Also, any tips on using dummies, and how far in advance I could decorate those as to avoid fading in coloring, etc. 

Thank you!!!!!!

The 3 tiered cake would give you enough servings.  So I would probably make the 3 tiered cake all from cake and then the two other cakes out of dummies.  That way when it comes time to cut and serve it will be easy to remember which is real and which isn't instead of spreading the real cakes out between all three cakes...know what I mean?

Yeah what she ^^^ said sounds right   Only thing I’m concerned about is finding such small dummies —the 4”era   

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You probably can ice & decorate the dummy cakes as much as a week-10 days in advance. Keep it in a cool, dark place to prevent fading.  Here's one I did much too early resulting in damage :(  https://www.cakecentral.com/recipe/7445/the-original-wasc-cake-recipe               I would make up ALL the icing you expect to need - make it batch by batch putting all those batches into one large bucket and mixing it well so there is one color for all.  Batches can differ slightly & that might show when used singlely.    Because styro is SO VERY light it can be difficult to ice.  Make up a 'dummy working board' by placing several long (several inches) nails in a board from the bottom.  Stick your dummy into those nails when working on it.                           

Thanks for all of the info! 

Kakeladi- I just see your WASC recipe with that link, I assume there is a different link? The tips are awesome, thank you so much! I love that about the batches of icing to make sure the color is the same. I am doing some tiers in fondant too, is that okay as far as the timeframe? Do I ice it with buttercream underneath like a regular cake, or use something else to get it to stick? Thanks for your help!

Yes, that link is for the recipe I developed yrs ago from another recipe.  It is know, used & loved world-wide.  To ice dummies w/fondant just very ightly spray w/water then add fondant OR you could use b'cream or piping gel if you want-- anything to help the fondant stick to the styro.  As far as time - yes, they can be done about 10 days ahead from my experience.  I've used fondant and b'cream finished cakes as displays in my bakeries for up to 6-10 months but I wouldn't want to give such an old cake to someone to eat!! :)  

I use Crisco to stick my fondant to cake dummies, just a pretty good smear all over.  Some of my collaboration pieces used cake dummies covered in fondant.  They last a long time.  Just keep them in low light place to prevent fading.  I order my dummies from Global Sugar Arts.  For sure the have the 4" x 4", not sure about a 4" x 6", doubtful.

Thank you! Very helpful information. I did find all dummy sizes needed at Global Sugar Art!