I think the driving force behind what you want to do is going to be more plant choice, budget and how much work you want to do.
you want to have a carpet, it is good to have nutrition at the root level in the substrate rather than in the water. Enough light, and CO2 where applicable if you are using enough light to need it.
For CO2, I would look to add another manifold onto the existing regulator if needed, just don't necessarily take the cheap way out, lots of the needle valves are cheap and wander over time. Or ship it all together, or use it on one tank for now... It is all good, just be sure to choose plants appropriately.
In your position I would likely pick a tank I could light easily and cheaply (I really like a 20L for this because of it only being a foot tall and works well with 24" lights) - out of anything (I would likely not start with the Hex tank I assume it is taller than deep and will likely be a pain to light effectively - remember watts per gallon is bunk and the important thing is measurable PAR at the place with the plants and the amount of PAR gets to be less as you get further away from the light fixture so deeper tank needs more light) , spend money there on substrate (you don't have to use all aquasoil, maybe just where root feeding plants are.. or good root tabs, API and Seachem aren't complete..), and run CO2 if you like.
Personally, most folks I know don't do EI (Tom.Barr only popularized it, didn't invent it by himself) they will either dose something custom they come to through trial, error and testing or will just use an all in one liquid for simplicity (Tropica or Thrive are most common) most are now honest enough to admit to not doing 50% water changes a week.
I think that is enough setup talk for now... Say if you have more questions or I ignored stuff.
For Rex, I am too lazy to look, but I want to say he was pretty much estranged from his family, unless I am thinking of the wrong person.
I would also suggest seeing if you can find a local aquarium club you can join, or even better a plant one.. its just a good source for cheap or free stuff.. not to mention like minded individuals you can see in person.. if possible inwould look into a trip to the AGA convention in Chicago at the end of next September... Possibly the AGA can help you find a club local to you.