ADon't use gumpaste! maybe that is why they're getting too hard?... use fondant but only with a pinch of CMC or tylose to make the ruffles. I think they will still get hardened but nowhere near inedible. Good luck!
Fondant will only stay soft and somewhat pliable once exposed to air if it is in contact with buttercream or ganache. On its own, it will air dry stiff and as far as I know you cannot do ruffles and expect them to stay that way without exposure to air. If you were to cover the cake the moisture from within the cake would cause the ruffles to droop.
When I make ruffle cakes i usually use a 50/50 combination of fondant and modeling chocolate. That keeps it softer because it doesn't totally harden up. But that style of cake is going to be a pain to cut no matter what you do.
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