A decent active substrate will certainly buffer your water to the desired ph level of below 7 and ideal of 6-6.5 for cbs. I'd also strongly advise you do go down the route of RO as this will allow you much more control over what you are adding back into your tank.
Sounds like your tank is around 50l, and if I were you maybe use x2 5l bags of whichever substrate you choose. I'm using Ebi gold at present and it's been very good, but there are other brands available such as shirakuru, HELP etc which will all do a similar job. The deeper substrate depth will help to improve buffering and should hold your water at the desired level. It's expensive stuff though at £20 a bag, but I'd really encourage you not to spoil the ship for tar at setup stage as you should reap the rewards once shrimp are settled. A much cheaper alternative is akadama bonsai soil, which will also buffer to a similar or even lower ph. Trouble is it looks a bit like baby poo! Personally I like the darker active substrates as they show off the shrimp and plants better IMO.
Oh, and the salty shrimp gh+ will enable you to adjust up your RO to the desired level at water change time. A TDS pen is a really useful tool for this....you can pick a reasonable one up for about £12 off eBay.
