I have a 65 gallon dirt tank. MGOP capped with black diamond blasting media. I am also EI dosing and running preasurized CO2. I would have to advise against capping the soil with eco-complete. I have eco in my shrimp tank and is tough to plant in by itself, I think it would not keep the MGOP from getting into the water and making a big mess. It's just too light of a substate for capping. Pool Filter Sand or any colors of the sandblasting media that is on the market will be great, just rinse very well before use. They make a blast media that is the same grain size as the black diamond but it is more of a "river rock" looking color.
When I first started my tank I did not dose EI and my stem plants just all wilted and died. All my heavy root plants did great of course. Now that I dose EI I can grow some stems.
Get ready to do alot of water changes the first month or so after setup. Also be prepared to heavily stock the tank when you first set it up. A step I wish I would have taken care of.