AQ, internally there are plenty of teams using the 7.1 version of the compiler.  But there are also folks using drops of the compiler we're currently working on.  One way to get your hands on a newer pre-release drop of the C# compiler (and tons of other bits) is through the tech preview drops (aka community drops).  We plan on releasing several more of those in the future and will hopefully find a way to widen the audience over time as well.  So if you're not able to get the latest community drop, you'll hopefully be able to get your hands on one of the future drops.  We're also working hard on getting the beta 1 release finished up.  We still have plenty of work left but when we're done you should be able to get a hold of that version. 

Regarding mixing versions of the compiler and the runtime, a version of the compiler is built against a specific version of the runtime.  The 8.0 version of the compiler will run against the 2.0 version of the .NET Framework.