Can I switch freely between SourceAnywhere for VSS and Visual SourceSafe as SCC provider in Visual Studio IDE integration?

Yes. But there is one exception:

In Visual Studio.NET, web projects must be created using Visual SourceSafe. For these projects, you can freely switch between SourceAnywhere for VSS and Visual SourceSafe.

If a web project is initially created using SourceAnywhere for VSS, only Visual Studio.NET with SourceAnywhere for VSS Integration can load the project. Visual Studio.NET with Visual SourceSafe Integration will not be able to load the project.