If you want really excellent suggestions and tips on how to cover a cake with fondant, I highly recommend you get Rose’s “Cake Bible.” She goes through the whole process, in detail, and offers tips and pointers on how to keep the fondant pristine white and how to make it adhere successfully to the cake. For example, she recommends that you first practice covering the cake by covering the cake pan (turned upside down and greased on the outside) as many times as it takes to get a smooth finish. As you know, you only get one shot with the actual cake—you can’t lift it off the cake and try again, you’ll end up with all kinds of crumbs in the fondant. It sounds as if you don’t need practice in this particular area, but “The Cake Bible” does discuss what to cover the cake with before the fondant is applied, and it discusses what kinds of cakes work with a fondant covering.
It sounds as if the powdered sugar frosting you’re using might not be as stable as a buttercream. I like powdered sugar frosting (I also like milk chocolate—I guess I have a real sweet tooth!), but I have noticed that sometimes powdered sugar frosting can go slack after a few hours, especially if the air temperature gets hotter. Another problem is that unless kept airtight, it dries and forms a crust on the outside, which might not be the best surface for applying fondant.
You really should check out “The Cake Bible” for frosting suggestions, especially those that work with fondant. On the subject of taste alone, if you did a side-by-comparison, you would find that shortening in frosting, as opposed to all butter, tends to have a greasy mouthfeel. Another thing about powdered sugar frosting is that it is grittier and sweeter than a satiny-smooth buttercream or ganache frosting. I’m not trying to tell you what to do—if you really love your frosting recipe, keep it! I’m just trying to open your mind to other possibilities, in case you are up for expanding your cake horizons. I grew up with powdered sugar frosting, I still make it for children’s cakes, but after trying several of the frostings in “Cake Bible,” my adult palate has awakened to ethereal possibilities! Many of my guests have been floored because they have never tasted anything like, say, a Mousseline Buttercream subtly perfumed with Grand Marnier, or a Dark Chocolate Ganache. (You mentioned you have “Heavenly Cakes.” I highly recommend the caramel ganache on a non-fondant chocolate cake—super yum!)
Let us know how things turn out. Good luck!