A lot of quality RO systems also include a Deionizer, mine is a 6 stage system.A couple of points.
RO systems dont change your water hardness. Thats because they dont filter out calcium or magnesium very well at all. Water hardness is just a measure of how much dissolved calcium and magnesium is in the water. So using RO doesnt have much if any effect on whether you need to add calmag or not.
I have a water softener, but do I really need an RO system, too? Three reasons why RO and water softening work well together.blog.watertech.com
To reduce hardness, you need a softener or deionizer. The problem there is softeners add tons of sodium to the water - which your plants wont like.
As far as adding cal mag or not in hydro, I have been doing that for ever - but - I judge how much based on how the plants look. Purple stems means you need more. Nice green stems mean you dont.
As far as nutes and additives - avoid anything derived from plants or animals. Use pure salts only.
Pre filter, 2 carbon filters, RO filter, and 2 DI filters. Only cost me $145