When using the Atlas feature in the print composer is there any way of using both the Margin around feature and a defined scale?
Since the feature used to set the view extent margin is the same feature used to create each atlas view, the scale will be dependent on the feature size. It's thus impossible to set a defined scale for features with greatly different sizes and have a constant margin around them. If you use a defined scale, either some map extents will appear with a wide margin or some map extents will only be partially visible in the composer. If you use margins, the scale has to be different for every page.
If your features are identical in size, it's then a choice to make. Either go with a defined scale if you want nice round numbers, or go with a margin if you need a specific view extent margin around the features.