MSDN should have information about whether a particular winrt API works in desktop or metro or both (there are some desktop-only winrt APIs, many metro-only winrt APIs and some APIs that work in both environments).

The last I looked, about 50% of the winrt APIs work in both environments (I'm not sure if that 50% number includes the XAML APIs or not though - none of the XAML APIs work in desktop apps).

No windows runtime APIs will work on Win7 - I'm not aware of any plans to support windows runtime apps on Win7.