First off, Tom, welcome to Tropical Fish Keeping forum.
Now to your question and then some other comments. I am not a fan of the enriched substrates. I have had Flourite (which is basically the same as Eco-Complete) in one tank for 15 months and been very disappointed. I would not waste the money. My plants in the gravel and sand substrate tanks are just as good, and I am still having to dose liquid fertilizers in the Flourite tank just the same as the others. Another issue is substrate fish; both EC and F are rough, and not recommended for corys, loaches, some other catfish. I can't speak for other types of enriched substrates that are now available as I've never used them. But my general suggestion would be to use a plain sand or fine gravel (if corys are intended, sand definitely).
Next comment concerns Excel. I do not recommend this or similar products. API CO2 Booster is 1.6% glutaraldehyde (rest is water). Seachem's Excel is the same but a slightly higher percentage. This chemical is toxic. It is used to disinfect medical and dental instruments and as a chemical preservative. Considered a hazardous substance, skin irritant, toxic if inhaled, etc. You can read more here (which comes from a link on the API website):
It kills cells, and is used in products that attack viruses and bacteria. So, it may well affect bacteria in an aquarium. It will kill some aquarium plants outright at normal dosages, Vallisneria is one. If dosed at high levels, it will kill fish.
What is your tank lighting? This is the crucial aspect of planted tanks.
If you intend tetra, I would suggest bogwood, either a few chunks of wood and/or branches. Rock is less successful with characins. You might want to check the photos of my Amazonian tanks, click the "Aquariums" tab below my name on the left, for some aquascaping ideas. And the method used in all those tanks is natural, no added CO2 and minimal light. You can read about this in the 4-part series "A Basic Approach to the Natural Planted Aquarium" stickied at the head of the Aquarium Plants section.