Okay, first let me say I am still a newbie and I am still learning.

I have a wedding cake that I have been asked to due for July 10th. The bride wants a 16" square, 12" round, 10" square and 8" round. I have used the SPS with an all round cake construction but didn't know if I could use it for this type of cake construction.

Any help would be appreciated.

Sure you can.

But the 10" sq will not fit on the 12" round.
And the 12" round will look tiny sitting on the 16" sq.

It'll all be lopsided and top heavy.

what would you just for the cake sizes? I need it to feed 300 people.

Do you have all the different size pans? What I usually do and find it to be a great help is to stack the pans upside down on top of each other as if they were cake. Then stand back and see the proportion and how it looks. You have to be careful with squares on top of rounds, as k8memphis said, the 10" square will not work well on the 12" round. Stack the pans and you'll see what she means.

For serving sizes, you can go by the Wilton serving charts or you can go by the one on Earlenescakes website if you want slightly larger slices.

Good luck, that's a big cake!

Yeah, it's a pain to try an put a square cake on top of a round. Remember, that the size of a square pan is measured on one side, but the diagonal of a square is quite a bit larger, so the corners will stick out if you don't have a big difference between sizes.
If you don't have all the pan sizes to mix and match to figure out what sizes you need, it's pretty easy to figure out the diagonal of a square:
Multiply the length of one side by the square root of 2.
Ex: For a 12" square, multiply 12" x 1.4142 (the square root of 2) = 16.97"
So you would need at least a 17" round cake to even FIT a 12" square on top.

For your particular cake, you could use a 16" square, 14" round, 9" square, and a 7" round.
According to wilton, that would give you about 260 servings (including the top tier), or 214 according to earlene's chart. Since that's not enough for 300 (are you SURE they are EXPECTING 300, and not just INVITING 300?!?), maybe they would consider letting you do an additional "kitchen cake" or satellite cake? If not, it might be pretty tough to figure out sizes that will give you the right # of servings, and not WAY too many, and still have the cake look proportioned. Good luck!!!

Oh, and to answer your original question, yes, you can definitely use the SPS for this type of cake! Just get a square plate for the square cake!

........ can definitely use the SPS for this type of cake! Just get a square plate for the square cake!.......

One doesn't even need to get sq plates A sq cake can go on a round plate as long as it is one a cake board. Just find a plate the same size like a 10" round plate will hold a 10 or even 12" sq cake just fine.