I tried to define a CompositeSymbol as a resource in .xaml, like:
<esri:SimpleMarkerSymbol Size="6" Color="Black"/>
<esri:SimpleMarkerSymbol Size="6" Color="Red"/>
This does not compile.
I think, this is because Symbols returns the generic IList<T> interface only. I think, I read somewhere that XAML can handle collections only, if they implement the non-generic version as well.
Actually, I tested it with an own dummy CompositeSymbol class. If I define Symbols as IList or List<T>, I can define it in xaml. If I define it as IList<T>, it does not work.
Maybe, the CompositeSymbol can be enhanced to support IList as well.