Here is a link to calculate how much substrate you need... http://www.plantedtank.net/substratecalculator.html
You can use Eco-complete or Fluorite straight, no mixing. I would recommend Eco-complete because it is so easy to use, just open the bag and pour it in the tank, no rinsing required. You can also mix these with other gravels, make sure to get the same size gravel so one does not settle.
I would recommend at least 3WPG for Hairgrass, you could get away with a little less however you may not get the growth you want. If you have over 2WPG then you need to inject C02 and your Hairgrass will do better.