Laterite will give your gravel iron, Eco-complete is said to raise pH. I've never used it but I have read many threads that say that. Why not get ADA Aquasoil. It's about the same cost as what you were going to do and will give you great plant growth.
If you want to use inert soil then you can fertilize the water column and use fert tabs (break them up and put them under the places where you will plant heavy root feeders like swords, crytps etc. It will work well. Whatever substrate you use make sure it's small grained enough to told the fine roots of the glosso. Riccia has to be held down with a net or something.
Both plants are high light. I'm not sure your 30 watts will be enough. Are the bulbs new? Is the light a good spectrum? Do you have a good reflector? Maybe someone else can speak to this.