It looks there are two options for working with different styles in different scales: using other (duplicate) layer or define scale dependant style.
- Duplicate the layer(s) you want to set different styles in different scale
- Customize the style for duplicated layer
- Update maps in Print Composer using appropriate layers
- (Most important) Use 'lock layers' option in print composer item proporties for your different maps. Export the map.
- Choose Rule-Based style for your layer(s)
2. Define scale range for the style
- Export image also using 'Lock layers' and 'Lock styles for layers' options
Choosing the option is up to you. However, I recommend using first option for creating comperable maps (e.g. colour and black&white maps) (pretty much similar task) and the second one for scale-dependant view.
P.S. I didn't get if you already have different layers for different scale (you mentioned you want to use symbols instead of fills, what may mean using points instead of polygons). In this case, you need just the 'Lock layers' option. I wrote solutions for more common tasks (with no changing geometry).