I have a recipe for a marshmallow buttercream that calls to use fluff...im not sure if it will crust or not... I'm wondering if a non crusting frosting could be used to make rosettes cover the cake...or if they would just slowly fall

Yes, people use non crusting b/c all the time and pipe flowers and buds all over the cake. Of course , keep out of the heat as much as possible. I take demos at ices conventions , showing this. They are using smbc, imbc, . Beautiful and very tasty.hth