Fondant & gumpaste decorations stick to buttercream & fondant with water. If it's a heavy decoration, I'll use "royal glue" (equal parts meringue powder & water) or piping gel.
Royal icing will "melt" on buttercream, but it takes a couple days. So I've secured my royal icing transfers to my cake with buttercream. I just do it within 24 hours of the event. And I've used royal icing flowers on buttercream cakes--again, I just place them within 24 hours of the event.
I've used buttercream icing on fondant, because I don't typically have royal icing on hand.
I haven't come across any real "no-no's". Except, don't cover your cake with royal icing or gumpaste--they get hard.
Edited: 04/25/2008 10:25 AM by Moderator
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