Just re-posting the question.. really no ideas?
Column charts, by default, wrap each and every series specified into a ColumnSet. This is what allows each of the columns to reduce it's overall width to allow each column to fit in the available single item space. When you add a line series to the ColumnChart, this series also gets wrapped in the same column set, which reduces the width of your columns.
To overcome this, use your base chart type as CartesianChart. Add as many line series (or other series) as you'd like. For each of your column series, you will now need to manually wrap them in column sets. You can see a quick example of multiple series types all drawing as expected on a single chart.
<mx:ColumnSeries xField="x" yField="y2" displayName="Series 3">
<mx:ColumnSeries xField="x" yField="y0" displayName="Series 5" >
this should solve your problem. There are also different types of column sets, like stacked, for more charting options, if you want to explore them.