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

as a mum I agree worktop is priority, but also there's extreemly little cupboard storage so I would stick with the pantry cupboard and add a freestanding ikea Varde unit like attached pic ( I would have preferred integrated dishwasher too and a bit of moulding/ shape to the door style in a perfect world ) where's the fridge supposed to go?

We have Caesarstone in Misty Carrera on our island. It has a slight warm gray hue, which works well in our kitchen, and it's very durable against us and our kids.

If you mean a seam? Maximum length they can make a piece without a seam is determined by the length of the slab available.