Hi there and welcome to Simply
Starting with the tankmates first, although I love clown loaches, I am not a fan of them in with discus, nor am I a fan of ottos either.
You may find the 12 rummy's look a bit lost in a big tank, adding a whole load more would look great imo.
Will or does the tank have a background? Generally in planted tanks we tend to go with darker backgrounds, this, with the plants can cause the discus to go dark, as they try and "blend" in with their surroundings, a lot will depend on which starin you get though. Also some discus do not do well in CO2 injected tanks, so be prepared for that as well, when you introduce the fish, start with the CO2 at a low level and gradually increase it over a few weeks abnd this will give you and the discus the best chance! I had my CO2 on a solenoid so when the lights went out so did the CO2.
As for the top layer of substrate a lightish coloured sand is great ime.
As you are planning on using CO2 then I think you will need the 6x54w T5's.
As long as the cannister filters are working they will be fine, the Rena is not a fave of mine but many do like them.
The main advantage of using inline heaters is there is less clutter in the tank, other than that it is personal preference imo.
Love that 'scape btw