What icing consistency works best for scroll work?
It depends on what kind of icing. If you're using royal icing, make sure it's loose enough to pipe, but stiff enough to not be runny or spread out. Test it out on a table top or parchment. Pipe to see if it holds it's shape. You can thin out royal icing with a bit of water or thicken it with powdered sugar. If you plan on using buttercream, an Italian meringue buttercream is the best to pipe with, aside from American buttercream.
Yes, the recipe used will make a difference but for the most past what the other poster said is right on.