I like Eco Complete fine for small tanks but you wouldn't need a whole bag so I'd say some nice black sand would be fine. Water sprite is perfect for Betta's and easy to grow. All you'll need is some Flourish Comprehensive to add weekly as a fert after water changes. You can use Flourish Excel daily if you want as it will increase plant photosynthesis. Micro chain swords grow well in these tanks as would Java Ferns, Wendilov is my favorite in small tanks, and Bolbitus. Anubia Nana Petite is good is you have a small branch or piece of DW in the tank.
In the Edge filters, there is a sponge, biomedia tubes, and a place for carbon which I use Purgen there instead.