People had all sort of problems when using Apple unsanctioned Clang binaries (problems when Xcode updates, other software gets confused if you add your new Clang to the system PATH). It is easy to get in trouble, better to avoid the problem.
In theory, yes, you should be able to run a different version of Clang side by side.
In theory, yes, you should be able to run a different version of Clang side by side.