Aside from Vallis and Elodea/Egeria I don't think there's much that won't thrive in soft water! I have lots of Swords, crypts, Nymphaea/Nuphar, Java ferns and various mosses all thriving in very soft water. As for stems I find Rotala wallichi, Myaca and cabomba thrive, but I'm sure most stems would too, I'm just not that interested in them, too much work!