I think any moss that will grow in the low TDS conditions of a zebra tank would be fine. If you look in my Tully Gorge thread, there are pics in there showing the moss in the zebra habitats. Looks similar to many exotic species such as peacock and Xmas.
I suspect that the basic chemistry behind why your RO may be acidic is because When CO2 dissolves in water, it forms a weak acid called carbonic acid. Normally chemicals in water such as carbonates react with the carbonic acid to neutralise it, But since in RO these chemicals have been removed, they cant neutralise the carbonic acid and therefore the pH is low (acidic). My understanding is that the drinking water section of the RO unit adds a chemical to neutralise the carbonic acid and therefore the pH is not acidic (I could be wrong but thats my understanding). Easy way to test if this is true is to measure the TDS from the RO unit with and without the water passing through the drinking water section.
If you are using remineralised RO and inert substrate, then I cant imagine that there would be many inorganic nasties (thinking heavy metals, ect.) so macropore should be enough to remove the chemicals of most concern (particularly nitrogenous wastes - ammonia, nitrite, nitrate). I tend not to use carbon in any of my shrimp tanks unless it is for a specific purpose (e.g. Removing leftover medicine) which is extremely rare.
In terms of adding wood, I added a tiny piece of driftwood to my last setup. The shrimp seemed to do well with it. I definitely wouldn't add alot as there isn't a great deal of large wood in their natural habitat, but there are lots of leaves and some twigs.
I dont know enough about the blackwater product you suggested to reccomend using it or not. I can reccomend using benibachi minerals as Squiggle used it with success, and I'm currently using it to remineralise RO for my zeb tank, and some other native shrimp tanks as well. I dont think the benibachi is a complete solution as I suspect it doesn't contain all the trace vitamins and minerals that the zebs may require to look their best and maintain their health over the long term. Can the food we feed the zebs provide these? I dont know for certain. Its something I'm currently looking into.
Inert substrate is a definite must. Its hard enough to maintain a low TDS in a zeb tank without adding the headache of minerals leaching from your substrate.