Ah. Kinda two different separate then.
I'm certainly no expert. But I'd hazard a guess that a few generations of captive breeding haven't altered our ray's genes to any significant degree so given you have rays already living in that water it seems highly likely WC rays will also be fine.
Hard water would cause me to be extremely cautious when receiving the rays, however - squirt-and-dump, for example, would concern me even more than usual.
RO will certainly soften water - that's what it does - but as I'm sure you know, by default RO has almost zero buffering capacity and you can't keep fish in pure RO. You need to either ease off on the flow control so that you're producing impure RO (I don't remember if that will work) or reconstitute the RO with a percentage of regular water. Working out exactly how to do that in combination with using the water you produce for an automatic drip system is where I'd be asking questions in the DIY section.
If you really want to soften your water you can certainly get the RO unit, mix it in a barrel/bin/buckets for now and move on to a drip system at your leisure.