Is it possible to have on one shelf local pool0 and remote pool1?
Till now I have a symmetrical full-mirrored fabric-attached metrocluster. Now the customer came with a great idea to use unmirrored aggregate. I would recommend to work on shelf basis - mirror the whole shelf to another shelf on the other site and use the whole shelf for unmirrored aggregate(I have 4 shelves per site). But they need different amount of space for unmirrored aggregate so it leads to the situation where pool0-local has to coexist with pool1-remote on one shelf. Is it supported/recommended... what trouble it can bring.
Yes, having node 1 pool0 and node 2 pool1 disks in one stack (and thus in one shelf) is supported. It is not recommended as per TR-3548 (search for "mixed disk pool configurations") but it's supported.
You might need to manually assign any replaced disks though, since the controllers can't always figure out by themselves which controller should get any new disks assigned (especially if you have >1 spare per node)