Ok here I go....

1. How much cake mix would it take to make an 11x15, a mini stand up bear, 1- 6 inch round and and about 4 mini cupcakes?

2. how much frosting?

3. If putting the mini stand up bear on the 6 inch round and then that on the 11x15 sheet, how much support do I need and how should I do it?

4. The last one, would it be possible to make a white or yellow cake and marble it with chocolate and strawberry?
And what kind of filling? I was thinking banana, kinda like a banana split?
And do you think it would be a good combination or not?
BE HONEST!

Thanks!

hmmm......

Haha well I would agree, because the banana with the other combinations just doesn't appeal to me either.
I agree with regards to support to, you could get away with the bear and the small round and that sheet cake could support it. However, to serve it is the real reason to board your cakes and once you board them, you do need to support them with dowelling so that the board won't stick and make a mess.
Personally, if you want to serve the bear separately, he needs his own board, then the 6 inch would need its own board, again if you plan to serve it separately.
So you will put your sheet cake on its own board, at least double boarding this cake, if not, triple boarding or using plywood or a cake drum or something like that for support. By double boarding, I do not mean those flimsy white or white and silver cardboards either, I mean corrugated with the corrugations going in different directions.
So you let the icing set, then you place dowels in the area of where you are going to put that six inch cake on its own six inch board, but leave a space in the middle of where it will go. So, 5 or six dowels will do. Place some icing sugar in that little area, I find also a circle of parchment paper cut that same size as the 6 inch board will stop any issues with sticking to the icing. So you place that boarded six inch cake. Now, you cut a board small enough to fit your bear. Mark the underneath side of this board in the centre and using a sharpened dowel, make a hole in the centre of this board before boarding your bear cake. Two or three at the most, dowels, again leave that space in the centre, some icing sugar, the parchment. Do not stack Mr. Bear on top yet.
Now this is a case where you could even use straws instead of dowels because the cakes are not large or heavy.
But if this cake will be moved, you are going to want to take a dowel and insert it through to the bottom board of the sheet cake. But you don't want to mess up Mr. Bear's head. So what you do is measure the height of the sheet cake and the boarded round 6 inch cake and the boarded bear. And cut your centre dowel to be slightly shorter than the sum of all three. Sharpen one end of your dowel using a pencil sharpener or a knife.
Ok, so figure out the centre point of your 6 inch cake. Insert the dowel into this cake so that it goes down to the board that the big sheet cake sits on. Now take your boarded bear and looking underneath, line up your centre dowel to the hole that you made in the board that the bear sits on and thread him through.
So now you have stabilized him should he require movement and the centre dowel won't be sticking up through the top of his head.
Hugs Squirrelly