Hi Amanda,
Welcome to UKAPS! I am not sure why nobody has got back to you on your post and the questions you have asked. It is possible that the answers are already here in one of the other forums, so it might be worth doing a bit of searching around in the forums. From my experience virtually everything anyone has ever wanted to know or find out has been discussed on the UKAPS site, the problem is there is so much information it can be difficult to find sometimes. This means people ask the same questions again, when the answers are already on here. This is particularly relevant to the subject of Algae, every new starter in aquascaping experiences the same problems with algae, but if they look hard enough the answers are already here.
That said.....getting back to you post...The Rocky Mountain scape sounds great and I would love to see some pictures when its finished. The seiryu stone and eleocharis acicularis work well together, and your lighting and substrate materials sound fine. Now comes the tricky bit, RO water, minerals and pressurised CO2. I like using RO water because of its purity, and I agree with re-mineralising the water with salts such as you describe. Personally I've always used SERA Mineral salts for this purpose, but then I wasn't keeping shrimp specifically. The issues are:-
1. CO2 alters the PH of your water, i.e. reduces it so it become more acid and softer, fish like this and so do plants.
2. Mineral salts buffer the PH back up again, I assume the sulawesi 8.5 is talking 8.5 PH? In which case you will be pumping pressurised CO2 into the tank to feed your plants, which might not be necessary if you are only using eleocharis acicularis; then when you do your water changes with RO water you will be buffering the PH back up again.
When I used a PH sensor on my CO2 kit every time I added the salts I could see the PH reading change and the CO2 would start kicking in like crazy, which meant using a lot of CO2 which is not that cheap.
So you need to ask yourself are you more interested in how your plants feel and look or your caridina dennerli shrimp. It is definitely possible to do what you propose with CO2 and mineral salts, just be aware of the potential fluctuations in PH which is not good for plants or shrimps.
Enjoy, and lets see some pics.
Steve.