Hey Mel, to make it really simple, it all depends on what you like and want to use. The size is the only real key when it comes to hloding them and letting the roots become established. I started with standard aquarium gravel and had mixed results with plants. Once I mixed in some of the smaller river run gravel my plants too off and I have not had a problem since.
The only time you need a more fancy substrate is if you are trying to grow a small root plant like HC or similar. These plants really require a much smaller substrate like Eco Complete or Turface so their roots can actually get a hold of the substrate. If you are only wanting to grow the normal plants, then a small substrate, like an average size of a BB will work fine. I have grown Red Melon Swords, Crypts, Pearlweed, Elodea, Anubias, Primrose and most other plants that my light levels will support. The large pebbles could stay but they will wind up on the surface with the smaller gravel.