You can keep shrimp with or without soil, I have personally kept shrimp with ADA soil, up aqua soil and bare bottom tanks for about 6 years now. They all breeding fine for me but using shrimp sand or soil will definitely make the shrimp color better. There is no right or wrong to this, its more about what you wanted to achieve. Personally I would highly recommend keeping shrimp with substrate because It makes the shrimp color better and provides more bacteria (Food) too.