Hi Sarah, and welcome!
Of Rose’s buttercreams, mousseline is the firmest and best suited to roses, etc. However, because it is an all-butter buttercream, it is still temperature-sensitive. If you are working in a warm room or have warm hands, try resting the pastry bag on a chill pack or rotating two bags (one in the fridge, one to work with, then switch). Some forum members make roses with mousseline, while others use Cake Bible buttercreams for frosting/filling and borders, but use a stiff powdered sugar buttercream just for roses. Whatever you’re making with buttercream or ganache, just remember that cooling it will firm it.
The chocolate fondant should work beautifully, but isn’t elastic enough to drape over corners- that’s why the cake on the cover has fondant in two pieces, so it doesn’t crack at the edge/corner. Rose recommends a store-bought chocolate fondant for one that is flexible enough to drape.
Hector has made the chocolate fondant, you could PM him and I’m sure he’d know more about it.