Many species are also easily adaptable to a wide range of water characteristics as well, especially when tank bred/raised. From my limited understanding about Discus, they used to be incredibly difficult to keep and most certainly required RO a while ago, but since tank bred varieties are becoming more common, that simply isn't the norm anymore. I'm not trying to convince you to not use RO, I was simply curious as to your reason because some people think they need RO for no apparent reason. But if your fish will thrive better in RO (which likely they will) then it's good that you're recreating their natural environment as best you can.
Have you decided on a unit yet? Some come with the float valve and others require you to purchase it separately. Also, in order to use a float valve, you'll want to make sure that the unit comes with an automatic shut-off valve already plumbed in.