Ok then, plant care checklist time.
: There are pros and cons to all. I feel this is a highly personal selection, with no best choice.
: The old WPG "rule" is applicable to T8 fluorescent tubes only. It never was that accurate, and with any other lighting type it's completely irrelevant. Forget you ever heard about it. I'd say your stated lighting selection will put you at medium-low light in a 20G. Enough to grow a wide selection of plants, including many carpets/lawns; yet not so high that the tank is prone to algae outbreaks if you don't keep everything under perfect control. 6,500K is also a good color temperature, both for plants and for overall appearance.
: A DIY CO2 system is a good choice to begin with. I do recommend you get a drop checker, which when filled with a 4° KH solution and a few drops from a pH test kit, will allow you to monitor tank CO2 levels. Getting the gaseous CO2 efficiently dissolved in the tank is a whole topic by itself. Alternately, at this lighting level you could supply sufficient carbon to keep plants healthy by dosing a liquid source, like Seachem Excel. But the amount of carbon this can provide is limited, and far less than plants can actually use with this lighting; so you will see far faster growth with CO2.
: Especially with CO2 in play, a planted tank needs more water flow than a non-planted one. CO2 moves 10,000 times slower in water than in air. Water is viscous and sticky. Leaves deflect flow. Combine all this, and if you could actually see the CO2 concentrations everywhere in the tank, you'd see a layer about 1-2mm over every leaf where CO2 is noticeably depleted. This is called the Prandtl Boundary. Plants are removing CO2 from this area faster than it can move in. Good flow is required to help penetrate it and move CO2 in faster, so that plants will gain the full benefits. Multiply the number of gallons by 10, and that's a good guideline for how many gallons per hour (GPH) you need to move. And it should be fairly uniform, with all leaves in the tank swaying at least slightly in the current.
: You've probably already read about these. The "macros", which plants require in greatest quantities - nitrogen (N), phosphorus (P), and potassium (K). And the "micros", of which only trace quantities are required by comparison, but are still essential; there are many, but iron (Fe) is a notable one.
Dry ferts are an excellent choice, economical in the long run, and your first order can last years. I get mine from Green Leaf Aquariums
, but there are other sources.
Many people choose to dose using the Estimative Index (EI). Plants always need a certain amount of all nutrients to be present for healthy growth, but fairly large excesses cause no harm. EI takes advantage of that property. With it, you always dose an excess, so that no nutrient is likely to be insufficient; then prevent excesses from building up to harmful levels via a 50% weekly water change. The initial dosing recommendations are "one size fits most", not "one size fits all"; it can and should be tweaked if found to be insufficient. There are also other, leaner dosing systems should you want to minimize water changes. But these require proportionally more skill and experience.
If you're on the same water supply as me, note that our tapwater is low in calcium and magnesium. Just enough for most plants, but certain ones do benefit from adding a bit more. A GH Booster premix like this one