Ive made many cakes, but I'm not sure about the request I have as I do not own petal cake pans. 

Will it work as a 6 inch round on an 8 inch petal, on a 10 inch round on top of a 12 inch petal? I have no idea. They need 100 servings with saving the top. By my calculations...if dimensions work, it will yield about 115 servings. I'm just not sure if they will stack atop right and have space for a border (even a smaller one) or overlap.  

I've never been asked for petal other than once and I carved it since it was a single cake, so I hate to buy the pans just to check dimensions. 

Thanks in advance!!!

I have the Wilton petal pans. The sizes are 6,9,12 and 15 inches. Not sure if you were looking at another brand. I took photos of different size combos so you can see what happens. Hope it helps.

The first one is 6 " round, 9" petal, 12" round and 15" petal.

This one is 6 inch round, 9" petal, 9 " round and 13" petal. The 9" petal scallop sits right on the edge of the 9" round.

This is 6" round on 9" petal on 10" round on 13" petal.

This is 6" petal on 9" round on 12" petal on 12" round. Not sure if you would like to see any other combos?

Sorry photo 2 and 3 should say 12" petal, not 13".

In all photos where scallop is close  to the edge of the round cake it sits on or round pan is close to the edge of the scalloped pan it sits on, you could still fit a small border like a beaded border.

Hope this helps.