Idle means the Airgroup does nothing. If on ground they wait at their spawn position, in Air they wait on 500m height cycling around the waypoint, if they spawn heigher they sink to 500m (ok i ve not tested it in the latest patches, with regular patch they do), if changed later via script (enable) the Planes circle around the last position and sink to 500m. You can change the idle status via script.
Sometimes the game 'forget' to change the Taskstatus, so the Planes don't attack their target if they should do it. Don't think it's depend on the Planetype but this should be tested.
In my early tests (maybe it changed with the patches) i've never see any difference between scramble enabled/disabled.