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...)
http://blog.renklipasta.com/genel/angry-birds-asabi-kuslar
I think a cupcake topper is too small to make out of RKT. I would definatley do fondant or modeling chocolate. I am picturing this in my head and they would be so fun to make and so cute to look at them all. You will have to post a pic when you are done.
Oh good idea kaylani, that must have worked great it's the perfect size.
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