Welcome bploeg! Another reefer attempting planted! You'll be converting that 120G soon!
Your plans sound good. The RO would work, but mixing it with tap may be a bit more beneficial. What are your tap water parameters? Your tap water may be ideal for a planted aquarium! Those minerals in your tap can be quite useful.
In an aquarium that small you can skip the CO2 injection, and go with a CO2 supplement, like Seachem's Flourish Excel. That should be a suitable carbon source for you.
The only flaw I see in your plan is the SAE. SAEs get big. 5-6" big in fact. The 12 nano cube is too small for an SAE. Keeping an SAE in a nano cube is like keeping an Oscar in a 10G!
Hope I was able to help, and we hope to see a lot of you out on the forum!