Royal icing will not hold up well at all on buttercream. Buttercream remains a bit “flexible” for lack of a better word. Royal Icing dries hard and ridgid. So when you try to move the cake the royal icing will break and fall off, just like goodiegoddessjennb said. I would pipe buttercream on buttercream. I would pipe American buttercream details on top of your swiss meringue, let the details crust up and then gently brush colour on them. I think that may work out better. I have piped American Buttercream on top of Italian Meringue Buttercream with no problems.
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