the shaders add depth and form to your flat surface. The main idea is to add the illusion of shape to your object with them. Yes you will need different sizes. The "waffle" type just adds some texture to your object, A flower petal compared to a ruffle in a banner for example. flower smooth, banner rough. However there are no hard and fast rules on how you use these tools most were designed and created by people who had a specific use in their style of art. so the rest of us unless we copy their style learn to adapt them. And to make our own sometimes lol.
Practice on every scrap you have and the bellies if you buy full sides, i hate doing the mundane type practice also so i just cut what comes to mind and see how it works. Lifters also make a big dif in the form of your objects and work in conjunction with your shaders to create the three dimensional look.
I also practice my designs at least once on scrap before committing to a project, this allows me to practice my tooling, see if the design works, test colors and finishes etc. When first starting one time i did probably a dozen roses before finally doing the project.