Yes, if you look at the last part of the Area Chart Type sample, you can see where we manually set a Shape style with semi-transparent fill to the two areas. You could do a similar thing to whichever series you add to the chart, and yes, multiple kinds of series can be added to a chart.
You also could do something like in our PaletteCustomColors QuickStart where you specify exact colors to use for the style selector. You would specify semi-transparent colors instead of opaque ones. That might be a cleaner solution.