Quite a lot of issues in your questions, but here goes.
Your readings (nitrate at 10-20 ppm) do suggest that the tank is cycled, but the store's test will confirm. Your substrate is fine, no worries there; many put a layer of gravel over the flourite, and they will mix together a bit but that is OK. When you vacuum the substrate during the weekly partial water change, don't go down in the gravel, just run along the surface to pick up the mulm, and it will avoid further mixing.
Some thoughts on pH and hardness (kH and dH). dH refers to general hardness, and kH to carbonate hardness; the hardness of the water has a buffering action on the pH, so they are related. Best thing is to go with what comes out of your tap, especially since in your case this is not too bad for a planted tank. A pH of 7.2-7.4 is OK, and the gH and kH numbers show slightly hard water. Most of your plants will do well. Trying to change the hardness could cause pH shifts which is very stressful to the fish in the tank. For info, here's a link to an article that explains this in more detail: Aquarium Water Hardness
The tank when it is biologically mature (usually after 3 months) will have a steady hardness and pH that will be somewhat affected by the natural biological processes in the tank, but the weekly partial water change works to keep all this stable and constant (relatively so).
Which brings me to the partial water changes. This is the most important aspect of tank maintainance. A pwc of 30-40% every week is in my view the best, but you can manage with 25% so long as it is every week. There is much more than just the pH at stake here. Nitrates will rise and the plants consume some of them but the pwc is the main way of diluting them, and this is very important to prevent shifts in the ph and stress on the fish. No filter can remove urine and waste products from the water, only the pwc can do this.
When I had a 55g I did the pwc with buckets and a small plastic hose, very cumbersome. You should invest in a Python or Aqueon Aquarium Water Changer; both are similar, being a plastic hose that connects to a water tap with an assembly unit that has a valve so you can syphon water from the aquarium using the water flow, and then reverse the valve and fill the tank. Makes the pwc a breeze; don't forget to squirt the water conditioner in the tank at the start of re-filling. You need a sink with a faucet that has threads, usually like a laundry room sink or kitchen sink. If this is not possible in your case, the other option is to use a pail with a short bit of plastic hose. Place the pail of fresh water slightly above the top of the tank (use a small step ladder, etc) and with the hose syphon the water into the tank. You have a smaller flow and can direct it away from the substrate.
Your fluorite substrate has nutrients but this usually has to be supplemented with liquid fertilizer. Plants require many macro- and micro-nutrients and in the correct balance, otherwise they will not grow well and in some cases will die off from too much or too little of certain nutrients. A balance liquid fertilizer ensures they get what they need in balance. Seachem's Flourish Comprehensive Plant Supplement is very good, as is Kent's Freshwater Plant Supplement. I've experience with both. Add the recommended dose weekly right after the pwc; sometimes you need twice a week, depending upon the plant's response.
Stay away from CO2 especially if you don't have time for water changes. The plants will require more maintainance if they have added CO2. The CO2 produced by the fish is sufficient for most rooted plants and some of the stem plants [these latter being faster growing generally require more CO2, nutrients and light]. Now we come to the light.
You have 112 watts of high output [T-5 tubes are high output] light. This will be plenty over a 55g. You will have to control the light and experiment to find the balance between how much the plants need compared to the available nutrients; if there is more light than the plants can use (and this is determined soley by the nutrients being CO2 and macro- and micro-nutrients) for photosynthesis, algae will take advantage and become a mess. T-5 light is very bright. Eight hours a day is the usual minimum for good plant growth, but you may be able to extend this to suit your viewing times; just keep an eye on the algae. If it becomes rampant you will really have tank maintainance. Here again, the weekly pwc of 30-40% has a big effect on algae and will help you.
Use a timer for the lights, as plants (and fish actually) are healthier with a natural regular light/darkness schedule. The 6700K tube is a good one; Kelvin is the measurement of the colour of the light, and 6500 is the sun at mid-day at the equator. Over a planted tank, tubes with a K rating around 6500 are generally considered the best.