MMF is about the perfect consistency/elasticity for me about 5 hours after I make it. I add a little cornstarch to mine. I haven't had any problems covering square cakes. I just form the corners diagonally from each other...
Part of it will depend on how tall your tiers are. I'd say that the serving chart above is planning on 4" tiers. If yours are shorter then they'll serve fewer....
Here's a great tutorial on stacking cakes:
http://www.youtube.com/watch?v=74k0ZopsCW8
You can find some really helpful videos on youtube!
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Elise
http://elisescustomcakes.weebly.com/
I personally like to paint my silhouettes. You can buy edible paint or make your own with gel food coloring, a little corn syrup and vodka.
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http://elisescustomcakes.weebly.com/