If it were me, I would use fondant for the whole topper. RKT is great, but it is hard to get a smooth finish as it's so textured. I also find it hard to cover ball shapes in fondant.
It will obviously take more fondant, but the outcome will be better, in my opinion. You could consider making 2D as opposed to 3D toppers to save fondant if that's a concern? (See the link below - these are toppers on cookies, but the effect would be the same if you slice the top of the cupcake off, cored the centre, filled with buttercream then added the topper...)