There are many examples on houzz of contrasting counters for perimeter and island. For example butcher block on island and stone on perimeter or a solid color on one and something with movement on another. I would only mix two stones if one had movement and one was fairly solid. The stainless on perimeter and something else on island sounds good to me. Here is a pic of stainless counters with what looks like a stone breakfast bar. (I hope I understood your question right). http://houzz.com/photos/3137660

~$400 including expedited shipping. My GC just had his regular countertop guy do them, which wasn't great.... he had trouble making clean cuts. If I were going to do it again I would have pressed for someone with expertise to cut them. You can see in the image that the facing is not made of the stone peak, because too much was broken during the cutting to make the mitered edge. I'm a little bummed, but mostly pleased with the surface (which is the important part) thus far.

How about butcher block instead? IKEA and the big box stores sell it for countertops, the price is decent, and you won't have as many issues with buckling, water damage, or what to do with the edge as you would with the ply. If the edges of the ply get wet, or if any seam around a sink gets wet, the plywood will "blow" and you will have to replace it all, anyway.

The materials will need to meet the requirements for the hearth for the woodburner you choose. They need to project a set distance forward of the firebox opening as well, unless you have a concrete floor or other fireproof material. The structure needed will also depend on the weight of the woodburner you choose. It will need to hold that weight and hold it against sideways loads for earthquake safety. If you want a thin looking one, you may need an engineer to design it.