I would say it would be better to do regular vacs than reply on the shrimp to feed on the leftover food on the bottom - keep in mind they are quite substantial little waste producers themselves. I just had my one single amano shrimp in a 4 gallon breeding tank this past week, and I'm amazed at the amount of poo ! even though she's only been fed some flakes once during that period, so she must have been snacking on the plant in absence of food.
They also really wouldn't do well in a tank without plants and algae.