I am making a sombre rosette cake and wanted to use fondant on top and droop it off edge of top to look like snow for my granddaughters birthday cake. My question is will it knock my rosette off or make them sag any suggestions please.

No, it won't. Make sure you pop the cake with the piped rosettes in the fridge to harden and just use a thin layer of fondant.

 The problem I see is that the edges of the rosettes will make the cake surface not appear smooth as you might want it to look. Since you won't see the rosettes underneath the fondant, I would rather leave the top smooth and only have rosettes on the sides.

 Same in case you wanna use poured fondant or royal icung - which might even look nicer to imitate snow and you could let it droop off the edge looking more natural.