Don't mess with the pH or hardness. Keep water quality up (lots of water changes) and they will all thrive. If you were trying to trigger breeding in the rummynose then yes, you may want to move them to their own setup so that you could safely soften the water. But that is not needed at all in this situation, especially with hardwater fish in the tank as well. It is better to keep the water quality up and the chemistry stable by not screwing with it and just doing normal water changes.