Do I need to use anything to attach fondant/gumpaste to buttercream, or will it stay put on it's own?
if your buttercream is still "wet" you should be fine... but putting a small dab of water with a paintbrush on the back of your fondant/gumpast decoration will secure it better.
You can also use a little dab of buttercream if the buttercream is dry on the cake.
For striping, or flat dots, etc, I paint the back of the fondant with a TOUCH of water and stick it on. For a flower that sticks out from the cake I use a dab of buttercream.