You may need to add some root tabs into the gravel for the sword and val since they are heavy root feeders.
I would say yeah buy a bag of it i probably would not use it all but probably half maybe a little bit more. Those vals would love to geta big root system on them they would do better. I run eco complete in my tank and i like it its a good sub. Just get a nice thick layer.Sorry for asking again but is 1/2 inch natural gravel enough or can I cap it with something like Eco complete I know it's suppose to be the other way but just want our plants to do good