Sure you can do it.
(At least to change the maximum username length).
Put this line somewhere in your qa-config.php file:
define('QA_DB_MAX_HANDLE_LENGTH', 30);
This overrides the standard value of the "QA_DB_MAX_HANDLE_LENGTH" pseudoconstant. Other pseudoconstants you can see in /qa-include/qa-db-maxima.php file.