When you're using ADA aquasoil, kh buffering agents will have absolutely no effect until the substrate has been completely exhausted, so it's no use. As far as I know, its effects on gh are not very significant. I do not use anything to remineralize with either when using RO water, yet my pH sits around 6.4, so I wouldn't worry about the aquasoil lowering it too much.
I don't really know a whole lot about the chemistry behind it, but I do know that aquasoil is simply designed to keep the water soft.
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How I feel about non-planted aquariums.