I just tested some rainwater that I had out back, pH 6.6, KH and GH both zero. If I ever get interested in breeding, I may collect rainwater!
I just had a long conversation with a guy that was in the fish store as I was shopping. He raises discus, among other things, and his friend who is just beginning a tank has a rainwater collection set up that has 3 barrels connected for settling out solids. We have so much air pollution here with the refining industry that I would think that one would also need to filter with carbon and chelating agents for heavy metals. You might not have that issue in your area, but the SkepticalAquirist site has some info on how to use rainwater. That might be your best bet.
I don't know the answer about too much K from the softener, but the K will be consumed by plants. I know others have posted that they do it successfully, but you would need to ask around to get any firsthand info, I'm just recalling what I've read.