1. Lights. I have WAY too much (hello algae bloom...). I originally wanted a reef tank, and my lights could easily provide enough for a 100g marine reef.
I will be setting the marine tank up over the next few months (hopefully!) so at some point I will change the lights on my freshwater tank, thinking T5HO. Does this sound like a good plan? Or should I look at still lower strength lights? Anything to beware of / specifically pay attention to? Anything I can do in the interim to keep my problems to a minimum?
Having excessive light does not cause algae, directly. Having high light, with insufficient CO2 and/or fertilizers, however, is a recipe for disaster.
You did not mention what lights you are currently using, but if they are emitting a lot of light, you would have needed to be injecting CO2 and dosing fertilizers. Your zero level of nitrates indicates you were not, which would have been a likely culprit for algae.
If you are switching lights, be aware that with high light, comes high maintenance (fertilizers, CO2, general plant maintenance such as replanting and pruning).
2. pH. I was reading that many plants will not survive an 8.0 pH because CO2 won't stay in a form that the plants can consume at this pH. I've been on another freshwater forum where the general - and very strong - consensus was to stick with the pH and don't do anything to change it lest it result in disaster. But can I have a (successful) planted tank with this pH? (Frankly, it's not very good for the fish either.) And if the consensus here is that it would be better to lower it, what's the best way? RO water? pH stabilizer?
Your pH is fine. CO2 will naturally bring down the pH as well.
The pH is probably fine for your fish, as most are now captive bred and adapted to local water conditions. As mentioned, if you want to lower it, using CO2 is a good way. RO water is also possible. I would avoid any kind of pH stabilizer product, as it will lead to wild pH swings that will ultimately be more harmful for your livestock.
3. CO2. Now I'm starting to get into the things I don't know too much about. Given my parameters, any suggestions would be welcome.
With an aquarium as large as yours, I would suggest that you look into investing in a pressurized CO2 setup. There are various commercial products that are available, but if you are handy, I would strongly suggest you try DIYing a setup yourself.
For more information regarding pressurized CO2, please take a look at my Primer to Pressurized CO2 (linked in my signature below).
4. Ferts. Ok, so let's talk about ferts. What ferts do plants need? Exactly what minerals and nutrients will I need to add, and what exactly should their healthy level be in my tank? I'd like to be able to monitor this, ideally, if not cost prohibitive, so any suggestions on that would be great as well.
Plants need a variety of nutrients (both macronutrients and micronutrients). The macronutrients that are provided are generally nitrogen, phosphorus and potassium. Micronutrients, such as iron, boron, manganese, etc are generally provided in the form of a trace mix (such as CSM+B).
Nitrates should be ~10 ppm (though this number can be as low as 5 ppm or as high as 20-30 ppm).
Phosphates are generally kept around 1-2 ppm, but again, this can be higher (high phosphates does not necessarily mean algae problems).
Potassium is generally not measured for, since it is provided in excess.
6. Substrate. If I remember correctly (...) I have Fluorite on bottom, with gravel on top. This is a year old, with no additional fertilizers.
This is fine. The Flourite on the bottom has a high cation exchange capability (CEC) so it will be good once you start dosing fertilizers.