Are Drush configuration files compatible across all Drush versions?
Drush configuration files, generally speaking, anyway, only set options for use in various Drush commands.  If an unknown option is used directly on the commandline, this will result in an error; however, Drush ignores unknown options that are set in a configuration file.  This makes it fairly safe to use the same configuration file for multiple versions of Drush.  I have been using the same config file from Drush 5 to present without an issue.

If you do end up with some configuration tricks that only work with specific versions of Drush, you can always make a drush8rc.php file; this will only be loaded by Drush version 8, and will be ignored by all other versions of Drush.