I like your ambitions for the tiny tank.
Of course all aquarium advise is best taken after a fourth and fifth opinion, but for my two cents heres what I say gives the best shot for an amano-class nano. PS, this is a mini book I just felt like writing today.
First of all the lighting:
I say literally put as much power compact lighting as you can fit and afford into the lid. If you will use one of the bulbs as a reef-ready combo bulb (where one-half of it is actinic, they arent that expensive for a 13-28 watter) you will love the lighting effect and the lower blue end is enhancing to your overall spectral quality. It'll make your greens electric.
for their combinations. Use three or four 28watt pc bulbs and fan the canopy somehow. You will like having more light than less, as your carpet growers need to stay compact in that small of a tank lest they grow tall and skinny. <----- he said lest.
CO2 injection would work well on that system. One bubble per second fed into the venturi input of a small powerhead is plenty. Ideally, time it so the cycle goes off at night. pH shift is notable if you are using relatively soft water (which for longevity you should be). Out of all the dosings available, I say add magnesium, iron and potassium. Use of a wide-spectrum supplement (I use Kent liquid fertilizer in my nanos) is okay if you are stocking lightly and feeding fish lightly. Don't overcrowd the water from week to week with dosings, fish bioload and food to them does 90% of the work when combined with bright lighting, IMO 20 watts is half what you need.
If you can find some of Amano's powersand off the net, or marine biosand for planted tanks, then you will be better off as these carry heavier organic loading which means you can fertilize less and change water often (this prevents any algae wars in your 7g) without disrupting plant nutrient uptake. Don't think that your plants have to get their fertilizer chiefly form the water column, in nature these are very diluted and much of the plant feeding is done through symbiotic relationships in the soil and its decaying matter (detritus-dirt). this is why in a clean Asian water way where you would find Echinodorus Tellenus (Sword plant) growing wild, you would not see tiny patches of beard algae growing on it unless the waterway was unnaturally rich (as in near a sewage runoff). We get beard algae in planted tanks because we run a water column that is too rich.
Aquatic plants can and do absorb nutrients in the water column through leaves, but it doesnt have to be nor is it the best way to deliver nutrients in an aquarium. Ideally, I like 70% support coming from the substrate and 20-30% coming in the form of liquid dosers. Why dont you see an amano tank with just flourite like everyone uses? Because he knows flourite does not compete with fine-grain organic material as far as plants go. The particles that collect in between flourite beds accumulate and pollute your tank slowly unless you siphon regularly. There is no siphoning in nature, there is only larger animals breaking down smaller ones. Siphoning a tank wrecks this process, and too much waste collects before it is naturally reduced. The best way is to have a dirt-particle sandbed that is slightly rich with once-living material, and change water regularly as it feeds the water column. The bed is slowly replenished by fish poop and broken-down food particles.
**Want to add that a flourite planted tank will also work (obviously very popular) its what I use because Im too cheapo to locate and pay for powersand. But in doing the regular siphonings I am also required to dose macro/micronutrients through the water column which feeds algae. AFter experimentation, a fine balance can be found that still works. I am only meaning that an ideal, easy to balance system would use the organic fine-particle substrate and it will work right off the bat with no experimentation in dosings/algae outbreaks. Heck, you can grow aquatic plants in zero substrate if you want to as this is how hydroponic gardens are designed. You will just be doing more water column dosing and with high light levels things get a little green coating on them sometimes.
if you are running high lighting and dense planting, adding your Fe, Mg, K and N lightly to the water column may be helpful as your plants are asking a lot in terms of nutrients in such as small tank. Thats why I like 20-30% use of liquid fertilizers.
Stock with 3-4 small fish like killies or something small, if you won't load the tank down to heavy you will need no filtration of any kind. Getting a CO2 setup would do wonders in the tiny ADA setup and it can be done well.
As the great gump would say:
Thats all I have to say about that.