Honestly I don't know lol. I have never used a all sand substrate to grow plants. Usually people use sand to cap off soil or other nutrient rich mixes. I don't see it hurting anything if you did mix it. But since you already have the 3 bags what else would you do with them? If you plan on setting up another tank soon, I would just save them for that.
Personally I would take out as much sand as I could and do all eco, but that's me. My local petco had bags of eco for 19.99 if you need some more to fill it up, so check a local petco. That price isn't to bad. The easier thing would be just to mix it.
On a second thought, with the lights and co2 your plants may need that extra nutrient rich substrate. Your plants would basically be relying on all water column dosing, which IMO isn't the best route. The substrate will give you that reassurance of nutrients that the plants take up root wise.