If you had hard water you would probably know from all the limescale deposits in your kettle!
So it sounds like your water is good for pretty much anything. Platies are a peaceful fish and they will live happily with most other peaceful community fish.
First think about if you want to get more platies or not. I originally planned on getting a mixed community of platies and guppies for my tank but I fell in love with the platies and now I have a tank full of them! I love watching the social interactions between them. They seem so much more relaxed and behave much more naturally when they are with others of their own kind, you get to see behaviours that you wouldn't otherwise get to see with a mixed community. I think the same probably applies to most other species of fish (aside from fighting fish!).
What sort of fish do you like? Shoaling fish, colourful fish etc?