1)No, you don't need potting soil under the gravel. If you are using regular gravel make sure it is a small natural gravel & use root tabs for the roots. Or you can use a gravel like Eco-complete or Florite that as a lot of nutrients already in it for the plants. Which ever one you choose make sure the depth is between 2-3".
2)Yes, you need more light. You need a least 2WPG to grow low light plants. So for 77G tank you will need 154 watts to get 2WPG. Also you will need to use plants bulbs(5500k-10000k).
3)A 2 liter DIY will not be enough for your size tank; plus it would cost to much in the long run. If you can, go for a pressurized C02 unit.
4) Try this link for fert info:
http://www.csd.net/~cgadd/aqua/art_p...izer_intro.htm