Of course you can fight scum with scum or rebels with rebels if your opponent agrees. There is no point why not. The initiative can be rolled or determined in any other way if need be. For that matter we rank initiative at equal pilots values in the following order: Imperials (trained military) - Rebels (ex-military, semi professional, mercenaries) - Scum (potentially of the same standard, but with more than a few wannabes...)