No, Yates...no don't need to cap substrate if you don't wish to. You are only capping the substrate with an inert material, like sand or gravel, to make it aesthetically pleasing to look at.
In fact, I wouldn't cap the Eco that you will be using due to the fact that it is already the near-consistency of sand. Topping it with more sand would not make too much sense to me, but you could if you wanted to. Capping it with gravel would probably make it look worse, but that is only my opinion.
Your substrate choices are perfect, and capping adds no real value to growing plants; it is "everything" under the cap that matters.
In fact, I wouldn't cap the Eco that you will be using due to the fact that it is already the near-consistency of sand. Topping it with more sand would not make too much sense to me, but you could if you wanted to. Capping it with gravel would probably make it look worse, but that is only my opinion.
Your substrate choices are perfect, and capping adds no real value to growing plants; it is "everything" under the cap that matters.