Use gum glue. Just add water to gumpaste and let it dissolve to form a glue. It holds fondant to fondant very well. If you are worried about the weight of the ruffles, make them in advance and let them dry.
They will be a lot lighter and easier to attach. The gum glue will hold wet to dry fondant, no problem.
Your topic says ruffles on buttercream ... so when attaching fondant to buttercream I use some piping gel just brush a small amount on the fondant & it will attach to the buttercream just fine.
If you decide to do the ruffles with buttercream wilton tip 127 to do basic ruffles or wilton tip 86 does a really nice ruffle boarder. I use tip 86 most
Hope this helps
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