If the plaster (and pebble tec is plaster-based) is new (< 1 year) you should use a nylon-only brush. I would continue using the nylon brush if you are getting good results with it. My M.O. is to use the tool that is the least invasive while still getting the job done. If you find you need a stronger brush, then I would step up to the combo brush - but only if you need to. Full stainless steel brushes are particularly useful if you get algae, especially black algae. But if you follow our methods, you should never have to worry about that.